Fix tail -f in capability mode.

We were not adding CAP_EVENT to input file capabilities, so kevent()
always failed with ENOTCAPABLE. tail implements a fallback mode to
poll the file in this case, so the failure was not apparent.

Reviewed by: emaste
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D22709