MFC r258871:

Properly report an error instead of panicing when user tries to create
LUN backed by non-disk device, e.g. /dev/null.

Sponsored by:	The FreeBSD Foundation
MFC r259182:

Fix handling for empty auth-groups.  Without it, ctld child process
would either exit on assertion, or, if assertions are not enabled,
fail to authenticate the target.

Sponsored by:	The FreeBSD Foundation