In scripts/mk-vmimage.sh, prevent incorrect usage() by
defaulting VMCONFIG to /dev/null, and additionally
ensuring VMCONFIG is not a character device before it
is sourced.

While here, be sure to exit if usage() is called.

This should effectively be no-op, but the usage() output
was discovered while investigating a larger issue.

MFC after: 1 week
X-MFC-with: r277458, r277536, r277606, r277609,

		r277836, r278118, r278119, r278206

Sponsored by: The FreeBSD Foundation