Update skin cvar from character select and compare against cvar when warping without a loaded file (resolves #1267, #1117, #1114, #1112)

Similar to !2156 (closed) but reimplemented in a way that also fixes #1267 (closed)

This fixes the following three regressions:

1. Warping to a map from the title screen set your skin to Sonic
2. Using the ingame level select set your skin to Sonic
3. It was impossible to switch to Sonic via devmode when in single player without manually changing the cvar first

I've tested forceskin in multiplayer and it still behaves as !2100 (merged) intended.