Move etc/shells to lib/libc/gen with getusershell(3).

Approved by:	will (mentor)
Differential Revision:	https://reviews.freebsd.org/D16467

etc/Makefile

@@ -47,7 +47,6 @@ BIN1=	crontab \
 	remote \
 	rpc \
 	services \
-	shells \
 	sysctl.conf \
 	syslog.conf \
 	termcap.small

lib/libc/gen/Makefile.inc

@@ -4,6 +4,8 @@
 # machine-independent gen sources
 .PATH: ${LIBC_SRCTOP}/${LIBC_ARCH}/gen ${LIBC_SRCTOP}/gen
 
+CONFS=	shells
+
 SRCS+=	__getosreldate.c \
 	__pthread_mutex_init_calloc_cb_stub.c \
 	__xuname.c \