Hi all,
now that packages by default don't contain mtree specs, I would like to
start disabling the mtree code in pkg_add as well and make it print a
warning. Reasoning is that most cases never cared about it and it means
that pkg_add is one step closer to being standalone.

The change does two things:
(a) Always print a warning when the package contains a mtree specification.
(b) Always keep the mtree spec around (currently removed after use).

This means that a user *can* run mtree manually when needed.

Joerg