-Fix intermission issue with FreeDoom introduced in r4446.

git-svn-id: http://odamex.net/svn/root/trunk@4456 b0bcef46-2027-0410-bd56-ab726e9a1d04
-FreeDoom and Ultimate FreeDoom now use gamemodes commercial and retail respectively. They are identified as gamemissions commercial_freedoom and retail_freedoom now.
-Revert r4456 as it is no longer needed.
-Add retail_bfg gamemode.
-Refactor the test for lost soul bounce correction into P_CorrectLostSoulBounce. It appears that lost soul bouncing gets corrected for both Doom & Doom 2 BFG Editions based on the source code.

git-svn-id: http://odamex.net/svn/root/trunk@4457 b0bcef46-2027-0410-bd56-ab726e9a1d04