Update the pkg-stage target to be more compatible with pkg-1.2:

 - Add a release-dvd.conf pkg(8) configuration file to override
   the default FreeBSD.conf configuration.
 - Remove architecture-specific pkg-stage.conf files, consolidate,
   and move their contents to scripts/pkg-stage.sh.
 - Use 'pkg -vv' to determine the ABI, which is used as the
   cache directory.

Prior to these changes, it would be possible for pkg-stage to fetch
conflicting binary packages from multiple repositories.

Tested against:	head@r260522, stable/10@r260522

MFC after:	3 days
X-Insta-MFC:	possibly
Sponsored by:	The FreeBSD Foundation