bectl(8): don't allow creation of boot environments with spaces

Boot environment datasets that contain spaces are not bootable.

When a user attempts to create a boot environment with a space, abort
the creation and print an error message.

PR: 254441
Reviewed by: allanjude
Differential Revision: https://reviews.freebsd.org/D30194

(cherry picked from commit 0e6549c8745049e3d6fba3ad748034de2d5cbd2a)