- Reset stable/10 back to -PRERELEASE status now that releng/10.4
has been branched.
- Update __FreeBSD_version to reflect the new -STABLE branch.
- Switch the pkg(8) configuration for the default installation and the
DVD image creation back to the latest set, i. e. revert r322737.

Approved by: re (implicit)

Switch the pkg(8) configuration for the default installation and the
dvd1.iso to use the quarterly set, i. e. 2017Q3, during the 10.4-BETA
phase.

Approved by: re (implicit)

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.

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

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