Updated Proxy's apps API.

1. Better stack handling.  Also restored technically unnecessary consts
for return values.
Updated Proxy's client API.

1. Just stack restore improvements.
Updated Proxy's friends API.

1. Stack improvements, plus returning CSteamID structs.
2. Also, at a glance in OllyDbg, it appears returning long longs works
correctly.  There should be no need for the struct work around.
Updated Proxy's userstats API.

1. Normal stack improvements, plus minor fixes.
Updated Proxy's utils API.

1. Stack fixes, plus minor fixes.