ifconfig: reject netmask and broadcast for inet6

We don't support setting netmask or broadcast address for INET6
addresses, and trying to do crashes ifconfig. Handle this the
same way as af_link, by rejecting attempts to configure these
parameters.

PR: 286910
Reported by: Hayzam Sherif <hayzam@alchemilla.io>
MFC after: 3 days
Reviewed by: zlei, kevans, des, cy
Approved by: kevans (mentor)
Differential Revision: https://reviews.freebsd.org/D50413