MFC r298893:
Provide an example to the kqueue man page, showing
a basic usage example. Although it is an
untypical example for the use of kqueue, it is
better than nothing and should get people started.

PR: 196844
Submitted by: fernando.apesteguia@gmail.com
Reviewed by: kib
Approved by: kib
Differential Revision: https://reviews.freebsd.org/D6082