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

lib/libutil/pidfile.c

@@ -196,7 +196,7 @@ pidfile_open(const char *pathp, mode_t mode, pid_t *pidptr)
 	}
 
 	if (cap_rights_limit(fd, cap_rights_init(&caprights, CAP_PWRITE,
-	    CAP_FSTAT, CAP_FTRUNCATE)) < 0 &&
+	    CAP_FSTAT, CAP_FTRUNCATE, CAP_EVENT)) < 0 &&
 	    errno != ENOSYS) {
 		goto failed;
 	}