Drop support for guessing an old-style class A, B, or C netmask from

Drop support for guessing an old-style class A, B, or C netmask from
a bare dot-notation IP address by counting trailing zero octets.
Instead, assume -host when neither -net nor -netmask nor -prefixlen is
given.  Error out when -net is requested but no netmask is specified.
This removes the last use of the IN_CLASS* macros from this program.
OK claudio@ kn@, and many agreed with the general direction during g2k18.