Week 5: be/libbe (create/destroy/mount/unmount)

[Kyle Kneitinger]

To leave as much time as possible for implementing recursive boot
environments, I've bumped up the priority on all of the standard operations
to be done by next week.

So far, creation of bootenvs (both from the active bootenv, or a compatible
snapshot), destruction of bootenvs, and mounting them are done.  In the
coming days I'll be working on rename, unmount, and jailing them.

Error handling/reporting keeps improving as well, but this week I'd like to
add an additional char buffer to the libbe_handle_t struct that can store
more detailed error messages when possible, such as the exact problem with
a name, etc.