jls, jexec support

[Joerg Sonnenberger]

Hi all,
the patch http://leaf.dragonflybsd.org/~joerg/jail.patch includes
the jail enumeration code and jail_attach. It's partly based on
FreeBSD's code, some parts are written from scratch.
The jail_attach.2 man page is from the original submit of Paul Herman,
I send him a mail off-list about the license.

Matt, could you have a look over the namecache related parts, I'm not
sure if they are correct. Is there a way to map a namecache entry back
to a path? We could avoid storing the path in struct prison and
reconstruct it on demand for jls.

Joerg