Fixed|libcommon|libdoom|libdoom64|libheretic|libhexen|g++: Compiler warnings (unused variables, signed/unsigned comparison)
Fixed|XG|libcommon: Error in XG stair build spread algorithm

Due to an old refactoring oversight the stair builder would fail to
determine the neighbor sector to spread to.

Plus some minor cleanup.
Fixed|Client|g++: Build error (missing include)