[BUGFIX][READY] monsters that are piled up combine strength(*) to bash things

*: only the primary basher (mob right infront of door) benefits
*: only monsters directly behind the primary basher add 100% str, others add 50%
*: this pools monsters up to 5 tiles behind the primary, and up to 1 tile on either side
ex: P: primary (100% str=2), 2: helper (100% str=2), 1: helper (50% str=2), 0: disqualified monster, -: wall, +: door
.0210
.021.
--21.
01P1.
--+--
.....
total = 13; 2 + (2*3) + (1*5)

For zombies vs door, the total str would typically be:
1 zed: 5
3 zed (shoulder to shoulder): 10
4 zed (3 sidebyside + 1 behind): 15
6 zed (3 wide 2 deep): 20
15 zed (3 wide 5 deep): 50

min_str 5 for a door, 10 for blocked or boarded, 15 for blocked and boarded seem to make sense
walls, chain link fences, and other stuff should have min_str raised too. (personally I like the thought of a sturdy chain link fence keeping normal mooks at bay unless they start piling up 2+ deep, ie walking dead prison )

Final commit:
3 zeds to bash door in, +1 for boarded +1 for blocked.