Removed extra attribute from reference headers.

1. G++ don't like no extra unused statements.
Added work around for returning a SteamID.

1. Winelib doesn't seem to compile returning 64-bit values correctly.
It's supposed to make an internal struct and return the pointer to that
in EAX, but instead returns the value itself in EAX.  The real win32
Proxy blows up accordingly.
2. Core, for the moment, now tracks the user's SteamID.
3. user.cpp returns a pointer to this location for it's API call.