ifconfig: make interface address without mask an error

In commit d8237b955528, as part of the deprecation of IPv4 address
classes, Mike Karels introduced a warning in ifconfig for setting an
address without a width or mask. The commit message says "This will
hopefully be an error in the future."

As the warning has been included in releases from FreeBSD 13.1 on, and
sufficient time has passed, turn this into an error.

Reviewed by: allanjude, olce, grembo, philip, gordon
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45585