Switch the DVD pkg(8) repository to 'release_3' now that
the 9.3-RELEASE package builds are complete.

This is a direct commit to releng/9.3.

Approved by: re (marius)
Sponsored by: The FreeBSD Foundation

Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC r260772:
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.

NOTE: WITH_DVD=1 is broken on stable/9, pending updates to the
pkg(7) code. The 'dvdrom' target on stable/9 is broken as-is,
so there is no harm merging this change now. I think.

Sponsored by: The FreeBSD Foundation

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