monsters moving other monsters (trunk only)

For now, the code is conditional on BARGETHROUGH
being defined, while it gets tested further. While behavior is
different with and without BARGETHROUGH defined, savefiles
are the same either way.

After this patch is applied, only the riders have the M3_DISPLACES
bit set, but the Wizard and Vlad probably should too. Any others?

include/config.h

@@ -335,6 +335,7 @@ typedef unsigned char	uchar;
 #if !defined(MAC)
 # define CLIPPING	/* allow smaller screens -- ERS */
 #endif
+#define BARGETHROUGH	/* allow some monster to move others out of their way */
 
 #ifdef REDO
 # define DOAGAIN '\001' /* ^A, the "redo" key used in cmd.c and getline.c */