Make the difficulty level moddable

[highlander1599]

The difficulty level is hardcoded in src/difficulty.cpp

void	setDifficultyLevel(DIFFICULTY_LEVEL lev)
{
	switch (lev)
	{
	case	DL_EASY:
		setDamageModifiers(120, 100);
		break;
	case	DL_NORMAL:
		setDamageModifiers(100, 100);
		break;
	case	DL_HARD:
	case	DL_INSANE:
		setDamageModifiers(80, 100);
		break;
	}
	presDifLevel = lev;
}

Some players want to have it easier, some more difficult. Make it moddable so that every player can set the difficulty at his own wish.

[Chewbakka-Wakka]

Since this was opened, if talking of the cam mode then we now have SuperEasy + Insane added where you can also switch in-game as you play via debug/cheat menus. Or set in the main menu prior to starting a game.

For MP vs bots, other changes have and will be made to better this situation.