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