bectl: push space-in-name check down into libbe

bectl: push space-in-name check down into libbe

This check was previously in `create` only, not applying to renames.  It
should really be applied at the libbe level, so that we can avoid
writing about this restriction over and over again.

While we're here: `bectl rename` always succeeds, even when it doesn't.
Start returning the error.

Reported By:	Christian McDonald <cmcdonald netgate com>
Reviewed by:	rew, jwmaag_gmail.com (earlier version)
Differential Revision:	https://reviews.freebsd.org/D34605