core: move several source files to src/shared

As, the files are used by many executables, fstab-generator, remount-fs,
machine-id-setup, and etc.
With this change, the total size of the relevant executables and
libshared slightly decreases.

Before:
```
$ ll systemd-fstab-generator systemd-remount-fs systemd-machine-id-setup systemd-shutdown systemd-nspawn systemd src/shared/libsystemd-shared-247.so
-rwxrwxr-x 1 watanabe watanabe 7577800 Jan  5 13:35 src/shared/libsystemd-shared-247.so
-rwxrwxr-x 1 watanabe watanabe 4608360 Jan  5 13:35 systemd
-rwxrwxr-x 1 watanabe watanabe  117240 Jan  5 13:35 systemd-fstab-generator
-rwxrwxr-x 1 watanabe watanabe   61576 Jan  5 13:35 systemd-machine-id-setup
-rwxrwxr-x 1 watanabe watanabe  853080 Jan  5 13:35 systemd-nspawn
-rwxrwxr-x 1 watanabe watanabe   70600 Jan  5 13:35 systemd-remount-fs
-rwxrwxr-x 1 watanabe watanabe  172624 Jan  5 13:35 systemd-shutdown
```
Total: 13461280

After:
```
$ ll systemd-fstab-generator systemd-remount-fs systemd-machine-id-setup systemd-shutdown systemd-nspawn systemd src/shared/libsystemd-shared-247.so
-rwxrwxr-x 1 watanabe watanabe 7658336 Jan  5 13:32 src/shared/libsystemd-shared-247.so
-rwxrwxr-x 1 watanabe watanabe 4523560 Jan  5 13:32 systemd
-rwxrwxr-x 1 watanabe watanabe   78288 Jan  5 13:32 systemd-fstab-generator
-rwxrwxr-x 1 watanabe watanabe   30984 Jan  5 13:32 systemd-machine-id-setup
-rwxrwxr-x 1 watanabe watanabe  840384 Jan  5 13:32 systemd-nspawn
-rwxrwxr-x 1 watanabe watanabe   39104 Jan  5 13:32 systemd-remount-fs
-rwxrwxr-x 1 watanabe watanabe  117160 Jan  5 13:32 systemd-shutdown
```
Total: 13287816

meson.build

@@ -1914,8 +1914,7 @@ executable(
         'systemd-fstab-generator',
         'src/fstab-generator/fstab-generator.c',
         include_directories : includes,
-        link_with : [libcore_shared,
-                     libshared],
+        link_with : [libshared],
         install_rpath : rootlibexecdir,
         install : true,
         install_dir : systemgeneratordir)
@@ -2925,8 +2924,7 @@ executable(
         'systemd-remount-fs',
         'src/remount-fs/remount-fs.c',
         include_directories : includes,
-        link_with : [libcore_shared,
-                     libshared],
+        link_with : [libshared],
         install_rpath : rootlibexecdir,
         install : true,
         install_dir : rootlibexecdir)
@@ -2935,8 +2933,7 @@ executable(
         'systemd-machine-id-setup',
         'src/machine-id-setup/machine-id-setup-main.c',
         include_directories : includes,
-        link_with : [libcore_shared,
-                     libshared],
+        link_with : [libshared],
         install_rpath : rootlibexecdir,
         install : true,
         install_dir : rootbindir)
@@ -3287,8 +3284,7 @@ executable(
         'systemd-shutdown',
         systemd_shutdown_sources,
         include_directories : includes,
-        link_with : [libcore_shared,
-                     libshared],
+        link_with : [libshared],
         dependencies : [libmount],
         install_rpath : rootlibexecdir,
         install : true,
@@ -3336,8 +3332,7 @@ public_programs += executable(
         'systemd-nspawn',
         systemd_nspawn_sources,
         include_directories : includes,
-        link_with : [libcore_shared,
-                     libnspawn_core,
+        link_with : [libnspawn_core,
                      libshared],
         dependencies : [libblkid,
                         libseccomp],

src/core/meson.build

@@ -1,16 +1,5 @@
 # SPDX-License-Identifier: LGPL-2.1-or-later
 
-libcore_shared_sources = '''
-        killall.c
-        killall.h
-        loopback-setup.c
-        loopback-setup.h
-        machine-id-setup.c
-        machine-id-setup.h
-        mount-setup.c
-        mount-setup.h
-'''.split()
-
 libcore_sources = '''
         apparmor-setup.c
         apparmor-setup.h
@@ -151,22 +140,12 @@ load_fragment_gperf_nulstr_c = custom_target(
         command : [awk, '-f', '@INPUT0@', '@INPUT1@'],
         capture : true)
 
-# A convenience library to share code with other binaries:
-# systemd-shutdown, systemd-remount-fs, systemd-machine-id-setup, …
-libcore_shared = static_library(
-        'core-shared',
-        libcore_shared_sources,
-        include_directories : includes,
-        dependencies : [versiondep,
-                        libmount])
-
 libcore = static_library(
         'core',
         libcore_sources,
         load_fragment_gperf_c,
         load_fragment_gperf_nulstr_c,
         include_directories : includes,
-        link_whole : libcore_shared,
         dependencies : [versiondep,
                         threads,
                         librt,

src/shared/meson.build

@@ -145,6 +145,8 @@ shared_sources = files('''
         json-internal.h
         json.c
         json.h
+        killall.c
+        killall.h
         libcrypt-util.c
         libcrypt-util.h
         libfido2-util.c
@@ -165,6 +167,10 @@ shared_sources = files('''
         logs-show.h
         loop-util.c
         loop-util.h
+        loopback-setup.c
+        loopback-setup.h
+        machine-id-setup.c
+        machine-id-setup.h
         machine-image.c
         machine-image.h
         machine-pool.c
@@ -175,6 +181,8 @@ shared_sources = files('''
         mkfs-util.c
         mkfs-util.h
         module-util.h
+        mount-setup.c
+        mount-setup.h
         mount-util.c
         mount-util.h
         net-condition.c

src/test/meson.build

@@ -775,8 +775,7 @@ tests += [
         [['src/test/test-umount.c',
           'src/shutdown/umount.c',
           'src/shutdown/umount.h'],
-         [libcore_shared,
-          libshared],
+         [libshared],
          [libmount]],
 
         [['src/test/test-bus-util.c'],