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

include/uapi/inotify.h

@@ -0,0 +1,29 @@
+// inotify(7) event masks, Linux's values.
+#ifndef UAPI_INOTIFY_H
+#define UAPI_INOTIFY_H
+
+#define IN_ACCESS        0x00000001
+#define IN_MODIFY        0x00000002
+#define IN_ATTRIB        0x00000004
+#define IN_CLOSE_WRITE   0x00000008
+#define IN_CLOSE_NOWRITE 0x00000010
+#define IN_OPEN          0x00000020
+#define IN_MOVED_FROM    0x00000040
+#define IN_MOVED_TO      0x00000080
+#define IN_CREATE        0x00000100
+#define IN_DELETE        0x00000200
+#define IN_DELETE_SELF   0x00000400
+#define IN_MOVE_SELF     0x00000800
+#define IN_ALL_EVENTS    0x00000fff
+#define IN_UNMOUNT       0x00002000
+#define IN_Q_OVERFLOW    0x00004000
+#define IN_IGNORED       0x00008000
+#define IN_ONLYDIR       0x01000000
+#define IN_DONT_FOLLOW   0x02000000
+#define IN_EXCL_UNLINK   0x04000000
+#define IN_MASK_CREATE   0x10000000
+#define IN_MASK_ADD      0x20000000
+#define IN_ISDIR         0x40000000
+#define IN_ONESHOT       0x80000000
+
+#endif

include/uapi/posix.h

@@ -354,6 +354,7 @@ struct statfs {
 #define AT_FDCWD            (-100)
 #define AT_REMOVEDIR        0x200
 #define AT_SYMLINK_NOFOLLOW 0x100
+#define AT_EACCESS          0x200
 #define AT_EMPTY_PATH       0x1000
 #define AT_NO_AUTOMOUNT     0x800