jail: introduce per jail suser_enabled setting

The suser_enable sysctl allows to remove a privileged rights from uid 0.
This change introduce per jail setting which allow to make root a
normal user.

Reviewed by: jamie
Previous version reviewed by: kevans, emaste, markj, me_igalic.co
Discussed with: pjd
Differential Revision: https://reviews.freebsd.org/D27128