Write test for userfaultfd()

[metan-ucw]

There is a demo code in the userfaultfd() manual page:
http://man7.org/linux/man-pages/man2/userfaultfd.2.html

[amannch]

Started working on this issue and published a patch to the mailing list.

[pevik]

Patchset from @ricardobranco777 #1280.