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-bin/check.c

@@ -63,14 +63,13 @@ cb_pkg_integrity(prop_object_t obj, void *arg, bool *done)
 int
 check_pkg_integrity_all(void)
 {
-	struct xbps_handle *xhp = xbps_handle_get();
 	struct checkpkg cpkg;
 	int rv;
 
 	memset(&cpkg, 0, sizeof(cpkg));
 	(void)xbps_pkgdb_foreach_cb(cb_pkg_integrity, &cpkg);
 	if (cpkg.flush) {
-		if ((rv = xbps_pkgdb_update(xhp, true)) != 0) {
+		if ((rv = xbps_pkgdb_update(true)) != 0) {
 			xbps_error_printf("failed to write pkgdb: %s\n",
 			    strerror(rv));
 			return rv;