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