Basic structure for dlopening libsteam_api.

1. Various naming/syntax cleanups.
2. SteamAPIContext now dlopen/dlcloses libsteam_api.so, which
is expected to be found in the root SteamBridge install dir
(~/.steam/root/steam_bridge).
3. Context no longer tries to create the root SteamBridge install
directory.  This is now required for operation.