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/package_remove.c

@@ -370,11 +370,7 @@ purge:
 	 * Unregister package from pkgdb.
 	 */
 	xbps_dictionary_remove(xhp->pkgdb, pkgname);
-	if ((rv = xbps_pkgdb_update(xhp, true)) != 0)
-		goto out;
-
 	xbps_dbg_printf(xhp, "[remove] unregister %s returned %d\n", pkgver, rv);
-
 	xbps_set_cb_state(xhp, XBPS_STATE_REMOVE_DONE, 0, pkgver, NULL);
 out:
 	if (pkgname != NULL)