Fix build on Windows, deprecate soup-portability.h

Fix build on Windows, deprecate soup-portability.h

soup-portability.h created portability problems (ha!) because the
windows headers do "#define interface struct", conflicting with
variable names elsewhere.

The only thing that needed soup-portability.h in the public headers
was soup-address.h, which uses struct sockaddr. But we can just do a
forward declaration of that type rather than actually pulling in the
headers. Then soup-address.h can stop including soup-portability.h,
and we can use gio/gnetworking.h in the handful of other places that
need networking-related includes.

This may possibly break some other modules that depended on implicitly
getting network includes as a side effect of including soup.h...

https://bugzilla.gnome.org/show_bug.cgi?id=692134