capsicum: allow the setproctitle(3) function in capability mode

Capsicum in past allowed to change the process title.
This was broken with r335939.

PR:           [[ https://bugs.freebsd.org/230584 | 230584 ]]
Submitted by: Yuichiro NAITO <naito.yuichiro@gmail.com>
Reported by:  ian@niw.com.au
MFC after:    1 week