Fix enemy solidity
!2318 (merged) introduced a bug where enemies would go through each other instead of acting as solid objects, a feature added in 2.2. This merge request fixes that by adding player and pushable checks to the MF_SPECIAL object checks in P_DoCheckThing that were removed in !2318 (merged).
next:
This MR (and 2.2.13):
Special thanks to Barrels O' Fun for finding this bug!

