'nethack --usage' and '?' menu

Write up a description of how the command line works on UNIX and put
that in new file dat/usagehlp.  Add support for
|nethack --usage | --help | -? | ?
to display it and exit.

Also add a menu entry for nethack's help command to show it during
play.  That can be suppressed by uncommenting new '#define HIDE_USAGE'
in config.h since it won't be useful on servers that don't give
players access to command lines.

New genl_display_file() just writes to stdout.  opt_usage(), which
calls it, might need some suid/sgid handling to make sure the output
is done as the player rather than as nethack.

doc/nethack.6 is already out of date again.

include/config.h

@@ -421,6 +421,15 @@
  */
 #endif /* CHDIR */
 
+/*
+ * Define HIDE_USAGE to keep the "description of command line" out of
+ * the help menu if players have no access to a command line or if that
+ * is radically different from the description for UNIX in dat/usagehlp
+ * (a better solution for that would be a separate file and different
+ * value for USAGEHELP in global.h).
+ */
+/* #define HIDE_USAGE */ /* */
+
 /*
  * Section 3:   Definitions that may vary with system type.
  *              For example, both schar and uchar should be short ints on