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.

src/sys.c

@@ -81,6 +81,10 @@ sys_early_init(void)
 #ifdef WIN32
     sysopt.portable_device_paths = 0;
 #endif
+
+    /* help menu */
+    sysopt.hideusage = 0;
+
     return;
 }