kernel: deprecate Internet Class A/B/C

Hide historical Class A/B/C macros unless IN_HISTORICAL_NETS is defined;
define it for user level. Define IN_MULTICAST separately from IN_CLASSD,
and use it in pf instead of IN_CLASSD. Stop using class for setting
default masks when not specified; instead, define new default mask
(24 bits). Warn when an Internet address is set without a mask.

MFC after: 1 month
Reviewed by: cy
Differential Revision: https://reviews.freebsd.org/D32708