btrfs-util: Move subvolume creation to basic/btrfs.h

Also make btrfs_subvol_make() an openat style function.

src/basic/btrfs.h

@@ -0,0 +1,9 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include <fcntl.h>
+
+int btrfs_validate_subvolume_name(const char *name);
+
+int btrfs_subvol_make(int dir_fd, const char *path);
+
+int btrfs_subvol_make_fallback(int dir_fd, const char *path, mode_t mode);