Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

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

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

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

Improve error handling and remove an unnecessary check on geom provider
type. GEOM provider names can't duplicate (or shouldn't -- devfs will either
break or only use the first one if they do) so using the first provider
by that name is a sufficient check. This also lets the scripted partitioner
install onto gmirror and geli and such things.

If no partition configuration is specified in {}, use the automatic setup
the interactive installer uses.

Example: bsdinstall scriptedpart ada0
Result: Use entire disk for a FreeBSD system with default partition layout

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