Fix write-locks (deny-all/exclusive flocks) in sopen()

SH_DENYRW (deny-all) locks were broken 18 years ago with commit b866f199.

This fixes that by treating SH_COMPAT the same as SH_DENYNO (only), rather
than the same as SH_DENYNO or SH_DENYRW (and performing no locking).

This is likely the/a root-cause of some SBBS file corruption on *nix systems.

src/xpdev/filewrap.h

@@ -106,7 +106,7 @@
 		#define SH_DENYWR   F_RDLCK    /* shareable lock */
 
 		#ifndef SH_COMPAT
-			#define SH_COMPAT			0
+			#define SH_COMPAT SH_DENYNO
 		#endif
 	#endif