- Copy stable/10@296371 to releng/10.3 in preparation for 10.3-RC1
builds.
- Update newvers.sh to reflect RC1.
- Update __FreeBSD_version to reflect 10.3.
- Update default pkg(8) configuration to use the quarterly branch.

Approved by: re (implicit)

MFC r293073:

- Use attach_md instead of hardcoding md(4) provider unit numbers
- Implement a gmirror_test_cleanup function, which in turn calls
geom_test_cleanup to clean up all md(4) providers allocated in the test
run.
- Remove duplicate logic in test scripts for removing md(4) providers.
- Don't create files in /tmp (outside the kyua sandbox); use the current
directory instead

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

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

- Make use of geom_subr.sh to improve regression tests.
- Keep class-specific data in conf.sh file.

Convert gmirror(8) regression tests to the new framework.