The `fanotify` API[0] is a linux-specific API for notification and interception
of filesystem events. In some ways it is similar to `inotify`, but with
different advantages/tradeoffs. It is particularly well suited to full
filesystem/mount monitoring (vs per directory) and for allowing/denying access
to files (`inotify` lacks this capability).

The `fanotify` API has been updated several times since it was enabled in Linux
2.6.37. Presently I've only included support for the original `fanotify`
features, and the `FAN_MARK_FILESYSTEM` addition made in Linux 4.20. There are
subsequent updates in 5.0 and 5.1 not covered in this initial commit.

This commit adds the relevant constants and types from
`uapi/linux/fanotify.h`[1] and two new functions (`fanotify_init`[2] and
`fanotify_wrap`[3]) to `src/unix/linux_like/linux/mod.rs`. While I believe this
API is also present on Android I have presently limited my attention to Linux.

Although this commit focuses on Linux 4.20.x's `fanotify` API/constants I have
skipped adding constants for `FAN_ALL_CLASS_BITS`, `FAN_ALL_INIT_FLAGS`,
`FAN_ALL_MARK_FLAGS`, `FAN_ALL_EVENTS`, `FAN_ALL_PERM_EVENTS` and
`FAN_ALL_OUTGOING_EVENTS` even though they are present in this kernel version's
headers. These defines were deprecated[4] in later releases with instructions to
not use them in new programs or extend them with new values. It would be a shame
for new Rust programs to use deprecated #defines!

[0]: http://man7.org/linux/man-pages/man7/fanotify.7.html
[1]: https://github.com/torvalds/linux/blob/d54f4fba889b205e9cd8239182ca5d27d0ac3bc2/include/uapi/linux/fanotify.h
[2]: http://man7.org/linux/man-pages/man2/fanotify_init.2.html
[3]: http://man7.org/linux/man-pages/man2/fanotify_mark.2.html
[4]: https://github.com/torvalds/linux/commit/23c9deeb3285d34fd243abb3d6b9f07db60c3cf4#diff-4c9ca62be6bf38cc08f7ea9daf16e379