man pages: deprecate Internet Class A/B/C

Mark functions inet_netof(), inet_lnaof(), and inet_makeaddr() as
deprecated, as they assume Class A/B/C. inet_makeaddr() mostly works
when networks are a multiple of 8 bits, but warn for anything other
than historical classes. Reduce other mentions of network classes.

MFC after: 1 month
Reviewed by: bcr, manpages
Differential Revision: https://reviews.freebsd.org/D32711