Add a DISABLE_MTREE option, which just disables processing +MTREE data
on package installation.

As requested by marino on IRC.  FreeBSD is going to have to keep MTREE
around for years yet, because of maintaining pkg-install(8)
compatbility.  DFLY doesn't have that limitation, but some development
work will need to be done to make the ports MTREE free.  This is just
to allow experimenting.
Forgot to adjust the date after editing pkg.conf.5
Add DISABLE_MTREE to the default config tests.