Simplify xbps_transaction_prepare()/commit() and related API changes.

- xbps_handle::transd -> new member with transaction dictionary.
- xbps_transaction_prepare: returns an int.
- xbps_transaction_commit: doesn't need any arg now.
- xbps_repository_pool_sync: doesn't need any arg now.
- xbps_pkgdb_update: removed xbps_handle * arg.
- xbps_transaction_missingdeps_get: removed, missing_deps array is in
  xbps_handle::transd("missing_deps") array object.

lib/repository_pool.c

@@ -181,8 +181,9 @@ xbps_repository_pool_release(struct xbps_handle *xhp)
 }
 
 int
-xbps_repository_pool_sync(const struct xbps_handle *xhp)
+xbps_repository_pool_sync(void)
 {
+	const struct xbps_handle *xhp = xbps_handle_get();
 	const char *repouri;
 	size_t i;
 	int rv;