Add variable FREEBSD_RELEASE_MAJOR_VERSION derived from FREEBSD_RELEASE_VERSION.
FREEBSD_RELEASE_MAJOR_VERSION will be 9, 10, etc.
Add knob for testing building with pkgng, commented out.
Use FREEBSD_RELEASE_MAJOR_VERSION variable.
Try to use PORTSLIST and PORTS variables, instead of hardcoding "ports.txt" for the list of
ports to build, and "p" for the name of the ports repository to use.
If ports are built with pkgng, an INDEX.bz2 file is not created.
We cannot use INDEX.bz2 for figuring out what ports to install.
Use the ports.txt generated during the build instead.

Help received from:
   Bryan Drewery <bdrewery@FreeBSD.org>
   Koop Mast <kwm@FreeBSD.org>
Add more logic for pkgng.
Just after we install packages in the chroot,
display what packages are installed.  We
do this before we delete all the package metadata.
If using pkgng, delete /var/db/cache/pkg to save space.