New jail related rc.conf variable

Hi,

while setting up a jail using rc.conf variables, I noticed that although
there were some variables controlling whether or not I want to mount devfs
or procfs, I did not found any simple way to use some nullfs or unionfs
mounts inside it.  Therefore, I though that the jail rc script was laking
of flexibility in sense that there is no simple way to do some unusual
configuration before starting the jail and I decided to add a
`jail_example_pre_exec' variable in rc.conf which is simply a command
to execute before jailing.
The scrawny patch is attached.

Regards,
-- 
Jeremie LE HEN aka ttz                                jeremie.le-hen_at_epita.fr
                                                                 ttz_at_epita.fr
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