Don't free behaviour trees when deleting all bots.

Also, rename the function to G_BotDelAllBots – “/bot del all”.
Set the trees pointer to NULL after freeing it.
Fix up BG_Alloc (debug mode): should call memset().
Do namelog cleanup after bot cleanup.
Merge branches 'master' and 'bots' into bots2

Conflicts:
	src/gamelogic/game/g_main.c