jail(8) should be able to set kern.securelevel

As described in man jail(8) the kern.securelevel in a jail can raised independent
of host securelevel. If the jail is run with a user other than root
(option -u or -U) this user can not change the securelevel. Therefore
the securelevel must raised before the user is changed and this can
be done in jail(8) only.

jail(8) needs an option to be able to raise the securelevel in a jail
if the jail is run with a restricted user. A workaround would be
a suid executable inside the jail, but this is refused due to
security constraints.

Fix: The following patch implements the option to raise the
kern.securelevel inside a newly created jail.

How-To-Repeat: Start a jail with other user (option -u or -U) and try to raise
the securelevel in the jail without any suid executable.