Change type signs of some members in doomdata.h, add USHORT/ULONG to m_swap.h...

Change type signs of some members in doomdata.h, add USHORT/ULONG to m_swap.h for setup code to use for getting sector/sidedef/linedef/vertex/etc numbers

I did this to attempt to fix a map so large it was acting very weird; sadly I have no idea if it's purely the number of stuff in the map or whether the nodebuilder mucks everything up anyway