Improved Bridge GetSteamID.

1. Also removed 0x%ps from Bridge/user, as this prints out a duplicate 0x
(oops).
2. GetSteamID now takes the SteamUser pointer and a pointer to where
to store the SteamID.  Research suggests that Winelib cannot, in fact,
correctly return structs, so this is the least painful way while avoiding
unnecessary uint64 usage.