Merge ^/projects/release-install-debug:

• Rework MANIFEST generation and parsing via bsdinstall(8).
• Allow selecting debugging distribution sets during install.
• Rework bsdinstall(8) to fetch remote debug distribution sets when they are not available on the local install medium.
• Allow selecting additional non-GENERIC kernels during install. At present, GENERIC is still required, and installed by default.

Tested with: head@r293203
Sponsored by: The FreeBSD Foundation