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.

(cherry picked from commit 20d59403961d531467cfab22163f49c131cc8b55)