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.
to deal with the fact that the public headers may be used with pre-c99
compilers, __restrict is used in place of restrict, and defined
appropriately for any supported compiler. we also avoid the form
[restrict] since older versions of gcc rejected it due to a bug in the
original c99 standard, and instead use the form *restrict.
