Expose explicit client auth string formats

This PR changes GetClientAuthString to act similar to the version in the 1.6-dev branch, deprecates it, and adds a new GetClientAuthString2 to allow explicit specification of the format wanted.

The reason for going with a new native is to not affect existing code (including blindly-recompiled code) while being able to keep a sane default (explicit rather than assumed Steam2). Additionally, it gives us the luxury of reordering params to be a bit saner.