inotify, faccessat2, fsync for Linux programs, and D-Bus credentials

What installing and starting Epiphany ran into:

- inotify: inotify_init/init1, add_watch, rm_watch, as a new kind of event
  descriptor. The filesystem reports creates, deletes, renames (with a
  cookie pairing MOVED_FROM and MOVED_TO), writes, close-after-write,
  truncation and attribute changes, to watches on the inode and on its
  directory with the name. ONESHOT, MASK_ADD, MASK_CREATE, ONLYDIR,
  DONT_FOLLOW, IGNORED on removal, Q_OVERFLOW, and Linux's coalescing of
  repeated events. The hooks cost one load when nothing is watched. Without
  it GLib found no file monitor and GNOME programs aborted.
- FIONREAD on inotify descriptors and pipes (inotifywait sizes its buffer
  by it).
- fsync and fdatasync had no Linux numbers since the first commit: GLib's
  atomic writes, gtk-update-icon-cache and dbus-uuidgen all got ENOSYS.
- faccessat2, the flags-taking faccessat musl uses for any flags.
- sendmsg accepts SCM_CREDENTIALS when they are the sender's own, as Linux
  checks; GDBus sends them in every handshake and was refused with EINVAL.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

kernel/fs/pipe.c

@@ -231,3 +231,13 @@ pipe_poll(struct pipe *p, int readable, int writable)
   release(&p->lock);
   return rev;
 }
+
+// Bytes waiting to be read, for FIONREAD.
+int
+pipe_avail(struct pipe *p)
+{
+  acquire(&p->lock);
+  int n = (int)(p->nwrite - p->nread);
+  release(&p->lock);
+  return n;
+}