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 r245700,245701,245702,245706,245707,245758,245759,245760,245792,245796,
245980,246013:

Automated installation support for bsdinstall.

Add a simple scripted partitioner. Documentation and more scripting support
will come soon. This lets the install process have a line like:

bsdinstall scriptedpart 'ada0 GPT {1.5G freebsd-ufs /, 10G freebsd-swap,
auto freebsd-ufs /usr}'

to set up a system with a 1.5GB /, some swap space, and a /usr using the
rest of ada0.

MFC after: 1 month