vfs: Initial revision of inotify

Add an implementation of inotify_init(), inotify_add_watch(),
inotify_rm_watch(), source-compatible with Linux. This provides
functionality similar to kevent(2)'s EVFILT_VNODE, i.e., it lets
applications monitor filesystem files for accesses. Compared to
inotify, however, EVFILT_VNODE has the limitation of requiring the
application to open the file to be monitored. This means that activity
on a newly created file cannot be monitored reliably, and that a file
descriptor per file in the hierarchy is required.

inotify on the other hand allows a directory and its entries to be
monitored at once. It introduces a new file descriptor type to which
"watches" can be attached; a watch is a pseudo-file descriptor
associated with a file or directory and a set of events to watch for.
When a watched vnode is accessed, a description of the event is queued
to the inotify descriptor, readable with read(2). Events for files in a
watched directory include the file name.

A watched vnode has its usecount bumped, so name cache entries
originating from a watched directory are not evicted. Name cache
entries are used to populate inotify events for files with a link in a
watched directory. In particular, if a file is accessed with, say,
read(2), an IN_ACCESS event will be generated for any watched hard link
of the file.

The inotify_add_watch_at() variant is included so that this
functionality is available in capability mode; plain inotify_add_watch()
is disallowed in capability mode.

When a file in a nullfs mount is watched, the watch is attached to the
lower vnode, such that accesses via either layer generate inotify
events.

Many thanks to Gleb Popov for testing this patch and finding lots of
bugs.

PR: 258010, 215011
Reviewed by: kib
Tested by: arrowd
MFC after: 3 months
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D50315