despite being practically deprecated, these functions are still part
of the standard and thus cannot reside in a file that also contains
namespace pollution. this reverts some of the changes made in commit
e40f48a4.

also move all legacy inet_* functions into a single file to avoid
wasting object file and compile time overhead on them.

the added functions are legacy interfaces for working with classful
ipv4 network addresses. they have no modern usefulness whatsoever, but
some programs unconditionally use them anyway, and they're tiny.