rc.d/jail patch for test/review

[Florent Thoumie]

This patch creates jail general options shared by all jails while  
keeping the ability to override the value on a per-jail basis  
(discussed with simon on IRC).

Example is:

jail_mount_enable=3D"YES"
jail_foo_mount_enable=3D"NO"

It also adds the ability to automatically add (and remove) the IP  
alias for each jail (jail_interface/jail_foo_interface).

http://people.freebsd.org/~flz/local/rc.d-jail.diff

Even if the patch is quite simple, I haven't tried it yet, so testing  
is more than welcome.

-- 
Florent Thoumie
flz at FreeBSD.org
FreeBSD Committer