Resolves #140 - doors/shutters now toggle based on the current state of ...

...the first door/shutter located in the loop.

This means you shouldn't have to press the button multiple times to open medbay doors (unless only one door is open and the other closed)
These changes also mean that blast-shutters no longer get out of sync with eachother. For instance, in rare cases one of the security lockdown shutters would be in the opposite open/close state to the others, rendering them totally useless.
Modified   code/game/machinery/door_control.dm