Add CAP_EVENT to pidfiles.

CAP_EVENT was omitted on pidfiles (in
pidfile_open()). There seems no reason why a process that creates
and writes a pidfile cannot monitor events on that file. This mod adds
the capability.

Reviewed by: cem@
MFC after: 2 weeks
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D25363