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-repo/main.c

@@ -241,7 +241,7 @@ main(int argc, char **argv)
 		if (argc != 1)
 			usage();
 
-		rv = xbps_repository_pool_sync(&xh);
+		rv = xbps_repository_pool_sync();
 		if (rv == ENOTSUP) {
 			xbps_error_printf("xbps-repo: no repositories "
 			    "currently registered!\n");