bectl(8): Add a "check" command

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