Proper sound of the chaingunner + behavior tweak file

This patch changes the chaingunner firing sound to the same as the player's and removes the extra tic for 'target check' that made ID give this enemy the shotgun firing sound Doomworld.com Now the chaingunner fires more smoothly and is a bit more lethal.

Credits: Myk who made and originally posted the patch in the aforementioned thread.

WARNING: this patch desyncs demos recorded without it and should be applied by all players for online games, as it alters the chaingunner behavior.