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/files.c

@@ -2577,6 +2577,9 @@ parse_config_line(char *origbuf)
         }
         sysopt.seduce = n;
         sysopt_seduce_set(sysopt.seduce);
+    } else if (in_sysconf && match_varname(buf, "HIDEUSAGE", 9)) {
+        n = !!atoi(bufp);
+        sysopt.hideusage = n;
     } else if (in_sysconf && match_varname(buf, "MAXPLAYERS", 10)) {
         n = atoi(bufp);
         /* XXX to get more than 25, need to rewrite all lock code */