libsys: fix sleep(3)/usleep(3) cancel behavior

Move functions back to libc/gen sources; they are only versioned from
libc and not libsys.
Access libsys interposing slots using __libsys_interposing_slot()
instead of direct __libsys_interposing array dereference, which cannot
work from libc.

Reported by:	glebius
Reviewed by:	brooks
Sponsored by:	The FreeBSD Foundation
Differential revision:	https://reviews.freebsd.org/D44042

lib/libc/gen/Makefile.inc

@@ -137,6 +137,7 @@ SRCS+= \
 	siglist.c \
 	signal.c \
 	sigsetops.c \
+	sleep.c \
 	srand48.c \
 	statvfs.c \
 	stringlist.c \
@@ -160,6 +161,7 @@ SRCS+= \
 	ulimit.c \
 	uname.c \
 	unvis-compat.c \
+	usleep.c \
 	utime.c \
 	utxdb.c \
 	valloc.c \

lib/libc/gen/sleep.c

@@ -56,7 +56,7 @@ __sleep(unsigned int seconds)
 	time_to_sleep.tv_sec = seconds;
 	time_to_sleep.tv_nsec = 0;
 	if (((int (*)(const struct timespec *, struct timespec *))
-	    __libsys_interposing[INTERPOS_nanosleep])(
+	    (*__libsys_interposing_slot(INTERPOS_nanosleep)))(
 	    &time_to_sleep, &time_remaining) != -1)
 		return (0);
 	if (errno != EINTR)

lib/libc/gen/usleep.c

@@ -46,7 +46,8 @@ __usleep(useconds_t useconds)
 	time_to_sleep.tv_nsec = (useconds % 1000000) * 1000;
 	time_to_sleep.tv_sec = useconds / 1000000;
 	return (((int (*)(const struct timespec *, struct timespec *))
-	    __libsys_interposing[INTERPOS_nanosleep])(&time_to_sleep, NULL));
+	    (*__libsys_interposing_slot(INTERPOS_nanosleep)))(&time_to_sleep,
+	    NULL));
 }
 
 __weak_reference(__usleep, usleep);

lib/libsys/Makefile.sys

@@ -40,7 +40,7 @@ SRCS+=	\
 SRCS+= getdents.c lstat.c mknod.c stat.c
 
 SRCS+=	creat.c
-SRCS+=	lockf.c sleep.c usleep.c wait.c wait3.c waitpid.c waitid.c
+SRCS+=	lockf.c wait.c wait3.c waitpid.c waitid.c
 SRCS+=	recv.c recvmmsg.c send.c sendmmsg.c
 
 NOASM+=  sched_getcpu.o