fs: implement inotify(2)

[gburd]

What

inotify was stubbed (inotify_init returned EMFILE, add_watch/rm_watch
returned EINVAL), so programs that watch the filesystem for changes could not
run.

How

Implemented against OSv's VFS. All path mutations route through a handful of
central functions in fs/vfs/vfs_syscalls.cc; those now call
osv_inotify_notify() with the absolute path affected and the event mask.
libc/inotify.cc keeps a registry of inotify instances (each a pollable
special_file, modeled on eventfd), matches every event against their watches,
and queues a struct inotify_event for the fd, waking any poller.

• inotify_init/inotify_init1 create the fd (IN_CLOEXEC/IN_NONBLOCK
  honored).
• inotify_add_watch resolves the path to absolute, adds (or merges, with
  IN_MASK_ADD) a watch, and returns a watch descriptor.
• inotify_rm_watch removes a watch and queues IN_IGNORED.
• read() returns as many packed inotify_event structures as fit in the
  buffer (blocking, or EAGAIN when non-blocking), rejecting a too-small buffer
  with EINVAL; poll() reports POLLIN when events are queued.

A directory watch fires for changes to entries within it (reporting the entry
name); a watch on the object itself fires with no name. Events wired:
IN_CREATE (mkdir, open O_CREAT), IN_DELETE (rmdir, unlink), IN_MOVED_FROM /
IN_MOVED_TO (rename), each with IN_ISDIR when the object is a directory. The
rename hook rebuilds the full destination path because sys_rename truncates
dest to its parent in place before the VOP.

Not yet covered (follow-ups): IN_MODIFY/IN_CLOSE_WRITE on writes (needs an
fd-to-path lookup), IN_ACCESS/IN_OPEN, and recursive watches.

Testing

tests/tst-inotify.cc covers create/delete/move on files and a subdirectory
(names and IN_ISDIR), IN_IGNORED on watch removal, and the EAGAIN/EINVAL
paths. Passes on OSv under KVM. tst-remove (unlink/rename/rmdir) continues to
pass, so the VFS mutation paths are unaffected.

(Recreated from #1422, which GitHub auto-closed when its branch was rebased onto current master. Same change, rebased and verified on master 3aba46c.)