'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doc/nethack.6 is already out of date again.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/extern.h

@@ -3303,6 +3303,7 @@ extern char *encglyph(int);
 extern int decode_glyph(const char *str, int *glyph_ptr);
 extern char *decode_mixed(char *, const char *);
 extern void genl_putmixed(winid, int, const char *);
+extern void genl_display_file(const char *, boolean);
 extern boolean menuitem_invert_test(int, unsigned, boolean);
 
 /* ### windows.c ### */