Use a POSIX lock for pkgdb and only issue pkgdb writes in exact points.

- Rather than using a POSIX named semaphore use a POSIX lock (lockf(3))
for pkgdb for writers. Writers that cannot acquire the pkgdb lock will
get EAGAIN rather then being blocked.

- Due to using a file lock we cannot write the pkgdb every time a package
is being unpacked, configured or removed. Instead pkgdb is only written
at the end of a specific point in the transaction (unpack, configure, remove)
or via xbps_pkgdb_unlock().

lib/initend.c

@@ -368,10 +368,6 @@ xbps_end(struct xbps_handle *xhp)
 	assert(xhp);
 
 	xbps_pkgdb_release(xhp);
-
-	if (xbps_object_type(xhp->pkgdb_revdeps) != XBPS_TYPE_UNKNOWN)
-		xbps_object_release(xhp->pkgdb_revdeps);
-
 	xbps_fetch_unset_cache_connection();
 }