More auth fixups for Steam auth

See the commit log for details. The goal is better supporting games that use Steam auth, no matter which format of Steam id that the engine uses as the NetworkID/AuthString.

Aside from clientprefs, I *think* this covers everything, if even indirectly.

Clientprefs is another beast that I don't really feel like touching. It still uses the engine's NetworkID, so worst cases are that prefs aren't shared between servers of different games or are lost upon the game changing format. The former is only relevant when storing in MySQL and may already be an issue across games. (I don't believe that it unifies STEAM_0/STEAM_1).