control of command-line usage entry in '?' menu

Instead of using a compile-time macro to suppress inclusion of the
menu entry to show UNIX command-line usage in the help menu, use a
sysconf setting instead.

Default is HIDEUSAGE=0, to include the entry for command-line usage.
Set HIDEUSAGE=1 to exclude that.  Does not affect 'nethack --usage'
if player actually has access to the command-line.

include/sys.h

@@ -50,8 +50,12 @@ struct sysopt {
     /* enable accessibility options */
     int accessibility;
 #ifdef WIN32
-    int portable_device_paths;  /* nethack configuration for a portable device */
+    int portable_device_paths;  /* nethack config for a portable device */
 #endif
+
+    /* nethack's interactive help menu */
+    int hideusage;      /* 0: include 'command-line usage' entry in help menu;
+                         * 1: suppress it */
 };
 
 extern struct sysopt sysopt;