bectl(8): Add a "check" command

This command simply returns 0 at the moment and explicitly takes no
arguments. This should be used by utilities wanting to see if bectl can
operate on the system they're running, or with a specific root (bectl -r).
It may grow more checks than "will libbe successfully init" in the future,
but for now this is enough as that checks for the dataset mounted at "/" and
that it looks capable of being a BE root (e.g. it's not a top-level dataset)

bectl commands can now specify if they want to be silent, and this will turn
off libbe_print_on_error so they can control the output as needed. This is
already used in bectl check, and may be turned on in the future for some
other commands where libbe errors are better suppressed as the failure mode
may be obvious.

Requested by: David Fullard
MFC after: 3 days