Acquire/release a POSIX file lock on repository archives.

- xbps_repo_open() accepts a third argument (bool) to acquire a POSIX file
lock on the repository archive.
- xbps_repo_close() accepts a second argument (bool) to release a POSIX file
lock on the repository archive.

This avoids the issue of multiple xbps-rindex(8) processes being blocked
even for different repositories on the same architecture, resulting in
unnecessary contention.

configure

@@ -340,40 +340,6 @@ echo "CPPFLAGS +=       -I\$(TOPDIR)/lib/portableproplib/prop" >>$CONFIG_MK
 echo "LDFLAGS +=	-lpthread" >>$CONFIG_MK
 echo "STATIC_LIBS +=	-lpthread" >>$CONFIG_MK
 
-#
-# Check for POSIX semaphores.
-#
-printf "Checking for POSIX semaphores ... "
-func=psem
-cat <<EOF > _$func.c
-#include <fcntl.h>
-#include <sys/stat.h>
-#include <semaphore.h>
-
-int main(void) {
-	sem_t *sem;
-
-	sem = sem_open("/xbps0000test", 0644, O_CREAT, 1);
-	sem_wait(sem);
-	sem_post(sem);
-	sem_close(sem);
-	sem_unlink("/xbps0000test");
-
-	return 0;
-}
-EOF
-if $XCC -pthread -lrt _$func.c -o _$func 2>/dev/null; then
-	POSIXSEM=yes
-	echo yes.
-fi
-rm -f _$func.c _$func
-if [ -z "$POSIXSEM" ]; then
-	echo "no! POSIX semaphores are required, exiting..."
-	exit 1
-fi
-echo "LDFLAGS +=	-lrt" >>$CONFIG_MK
-echo "STATIC_LIBS +=	-lrt" >>$CONFIG_MK
-
 #
 # Check for vasprintf().
 #