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

[kevans]

Summary

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>
Differential Revision:  https://reviews.freebsd.org/D34605

[kevans]

Reverse course, push the naming restriction into libbe as well for others' benefit (suggested by Christian).