Update the kqueue file monitering backend.

This update is a backport of work done by Martin Pieuchot (mpi@openbsd).
And will be available in the next major Glib released in September. This
update simplifies the backend and fixes a number of races and other
issues in the old backend.

Many thanks to Martin Pieuchot mpi@openbsd for the rewrite!

PR: 226920
Submitted by: Ting-Wei Lan <lantw44@gmail.com>, lightside@gmx.com
Obtained from: glib upstream