Lua fixes

Miscellaneous Lua-related fixes:

• P_IsFlagAtBase now works in HUD hooks
• Four new global variables are accessible to Lua: redflag, blueflag, rflagpoint and bflagpoint to Lua. The first two are mobj_ts for the red and blue CTF flags, and the last two are the mapthing_ts for their spawnpoints.
• skin_t variables should now be archived/unarchived properly in netgames.