jail: add security.jail.mlock_allowed

when the parameter allow.mlock was added a way for jails to check
if the parameter was set or now has not been added, this change
covers it.

MFC After: 3 days
Reviewed by: jamie@
Differential Revision: https://reviews.freebsd.org/D43314

(cherry picked from commit 9fd978680db6495180a4d46f76529d344b003c80)