libc: Add inotify support

inotify_init() and inotify_init1() are implemented using __specialfd(2).
inotify_add_watch() is implemented in terms of inotify_add_watch_at(2).

Reviewed by: kib
MFC after: 3 months
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D50315