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().

bin/xbps-pkgdb/main.c

@@ -72,7 +72,7 @@ change_pkg_mode(struct xbps_handle *xhp, const char *pkgname, const char *mode)
 	else
 		usage(true);
 
-	return xbps_pkgdb_update(xhp, true);
+	return 0;
 }
 
 int