mint-arena: Have bots try to crouch in BotMoveInDirection

This patch adds the code that is needed to get bots crouching in front
of obstacles where the bot should crouch. Here are the details what I
have done:

1. I delete the dead code in 'BotCheckBlocked'. Moreover I deleted the
   'if (movetype != MOVE_CROUCH || ...' condition a few lines down,
   because it did nothing.

2. Instead I added a new function called 'BotCheckBarrierCrouch' where
   it should be, called from 'BotWalkInDirection'. Technically
   'BotCheckBarrierCrouch' acts similar as 'BotCheckBarrierJump' apart
   from the fact that it checks for obstacles where a bot can crouch
   through and not obstacles for jumping. So this isn't really a new idea.

3. Inside 'BotCheckBlocked' I deleted the line
   'maxs[2] -= 10; //a little lower to avoid low ceiling'
   to get rid of the problem mentioned above.

4. Inside 'BotWalkInDirection' I moved the code for:
   '//get the presence type for the movement!'
   a few lines down. The reason: a few lines above 'BotGapDistance' sets
   'type |= MOVE_JUMP'. I think it's good to get the presence type after
   the movetype has been set, and not before! (though this is a safety
   thing, and not directly related to get crouching bots!)
mint-arena: Fix bot presence type for barrier crouch

There use to be a lot of crouching and standing up before reaching the shorter arena.
I think it's because BotCheckBlocked uses presence type and thinks move is blocked.
Setting presence type to crouch when crouching under barrier fixes it.

Fixes Crash getting stuck. Just kept crouching and standing up.