We really just want to skip this when building for Windows. So do that,
and remove the two header checks (we also already use both of these
headers, unguarded, in the !windows part of the codebase).
Squash the two *iffaddrs defines into one, as I haven't seen an
iffaddrs.h that implements one, but not the other.
Instead of a `gethostname` hack, use the official way of calling
`GetAdaptersAddresses` to get local network addresses on Windows.
As additional cleanup, move out `FromSockAddr` from MacOS and use it
everywhere appropriate.
Suggested by Ava Chow.
In almost all cases (the only exception is `getifaddrs`), we know the
size of the data passed into SetSockAddr, so we can check this to be
what is expected.
