winhttp(0/5): add preliminary IPv6 support

[Juan Lang]

Hi, this small patch series adds preliminary IPv6 support to winhttp.
I say it's preliminary because 1) it's not well tested, and 2) it
doesn't attempt to parse IPv6-specific address components such as the
scope ID.  Mainly I wanted to get rid of the assumption that every
host address is represented by a struct sockaddr_in, because the
underlying library functions are more general than that.

I'd appreciate feedback.  For instance, it may well be that this
doesn't build on all systems, and that more configure magic will be
necessary.

Cheers,
--Juan