This adds mount.linsysfs, mount.linprocfs, allow.mount.linsysfs, and
allow.mount.linprocfs to jail framework. By setting these values
in jail.conf (or as a sysctl), it is possible to have these filesystems
mountable within a jail.
This allows to nest a Linux jail inside of a native FreeBSD jail.
It is useful to do in order to run (and hack on) a jail management framework
that supports Linux jail while jailed itself.
The motivation is to make development and testing of Docker support in
FreeBSD easier and safer, but it is good for other things as well.