journal: add inotify watches by-fd instead of by-path

This is slightly nicer, since we actually watch the directories we opened and
enumerate. However, primarily this is preparation for adding support for
opening journal files by fd without specifying any path, to be added in a later
commit.

src/basic/fs-util.h

@@ -72,3 +72,5 @@ union inotify_event_buffer {
         struct inotify_event ev;
         uint8_t raw[INOTIFY_EVENT_MAX];
 };
+
+int inotify_add_watch_fd(int fd, int what, uint32_t mask);