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doc/nethack.6 is already out of date again.

src/pager.c

@@ -32,6 +32,9 @@ static void dispfile_optionfile(void);
 static void dispfile_optmenu(void);
 static void dispfile_license(void);
 static void dispfile_debughelp(void);
+#ifndef HIDE_USAGE
+static void dispfile_usagehelp(void);
+#endif
 static void hmenu_doextversion(void);
 static void hmenu_dohistory(void);
 static void hmenu_dowhatis(void);
@@ -2355,6 +2358,14 @@ dispfile_debughelp(void)
     display_file(DEBUGHELP, TRUE);
 }
 
+#ifndef HIDE_USAGE
+static void
+dispfile_usagehelp(void)
+{
+    display_file(USAGEHELP, TRUE);
+}
+#endif
+
 static void
 hmenu_doextversion(void)
 {
@@ -2389,6 +2400,7 @@ static void
 domenucontrols(void)
 {
     winid cwin = create_nhwindow(NHW_TEXT);
+
     show_menu_controls(cwin, FALSE);
     display_nhwindow(cwin, FALSE);
     destroy_nhwindow(cwin);
@@ -2411,6 +2423,9 @@ static const struct {
     { dokeylist, "Full list of keyboard commands." },
     { hmenu_doextlist, "List of extended commands." },
     { domenucontrols, "List menu control keys." },
+#ifndef HIDE_USAGE
+    { dispfile_usagehelp, "Description of NetHack's command line." },
+#endif
     { dispfile_license, "The NetHack license." },
     { docontact, "Support information." },
 #ifdef PORT_HELP