cleaning up INET classes: first batch of reviews

From: Mike Karels <karels_at_freebsd.org>
Date: Thu, 28 Oct 2021 17:37:31 UTC
I started a series of reviews for changes to clean up old code relating
to Internet Class A/B/C usage, including man pages that mark routines
as deprecated.  Most of them have no reviewers yet; please feel free to
add yourselves and/or comment.

The kernel and ifconfig changes add warning messages when an Internet
address is added without a mask.  The ifconfig changes were not straight-
forward, as there was no single spot that had all the relevant information.
The kernel changes add a new default mask (24 bits) and use it when setting
an address without a mask, replacing the code to check the network class.

kernel		https://reviews.freebsd.org/D32708
ifconfig	https://reviews.freebsd.org/D32709
libc manpages	https://reviews.freebsd.org/D32711
res_init.c	https://reviews.freebsd.org/D32712
mountd		https://reviews.freebsd.org/D32713
netstat		https://reviews.freebsd.org/D32714
sockstat	https://reviews.freebsd.org/D32715
sendmail	https://reviews.freebsd.org/D32716

Thanks,
		Mike