syscalls.master: allow vfork(2) in capsicum(4) capability mode

There is no reason not do do this, we already allow fork(2),
and I need vfork(2) for CHERI process colocation.

Reviewed by: brooks, emaste, oshogbo
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D39829

(cherry picked from commit 050555e129789fa2e20eba565f1320936b3d66e4)