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.

bin/xbps-uhelper/main.c

@@ -389,7 +389,7 @@ main(int argc, char **argv)
 			}
 		        xh.pkgdb = prop_array_copy(array);
 		        prop_object_release(dict);
-			rv = xbps_pkgdb_update(&xh, true);
+			rv = xbps_pkgdb_update(true);
 			if (rv == 0) {
 				printf("Migrated regpkgdb to pkgdb "
 				    "successfully.\n");