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doc/nethack.6 is already out of date again.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

@@ -26,6 +26,7 @@
 #define LICENSE "license"       /* file with license information */
 #define OPTIONFILE "opthelp"    /* file explaining runtime options */
 #define OPTMENUHELP "optmenu"   /* file explaining #options command */
+#define USAGEHELP "usagehlp"    /* file explaining command line use */
 #define OPTIONS_USED "options"  /* compile-time options, for #version */
 #define SYMBOLS "symbols"       /* replacement symbol sets */
 #define EPITAPHFILE "epitaph"   /* random epitaphs on graves */