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doc/nethack.6 is already out of date again.

sys/unix/Makefile.src

@@ -1191,7 +1191,7 @@ $(TARGETPFX)vision.o: vision.c $(HACK_H)
 $(TARGETPFX)weapon.o: weapon.c $(HACK_H)
 $(TARGETPFX)were.o: were.c $(HACK_H)
 $(TARGETPFX)wield.o: wield.c $(HACK_H)
-$(TARGETPFX)windows.o: windows.c $(HACK_H) ../include/wintty.h
+$(TARGETPFX)windows.o: windows.c $(HACK_H) ../include/dlb.h ../include/wintty.h
 $(TARGETPFX)wizard.o: wizard.c $(HACK_H)
 $(TARGETPFX)worm.o: worm.c $(HACK_H)
 $(TARGETPFX)worn.o: worn.c $(HACK_H)

sys/unix/Makefile.top

@@ -91,7 +91,7 @@ LUA_VERSION = 5.4.4
 # end of configuration
 #
 
-DATHELP = help hh cmdhelp keyhelp history opthelp optmenu wizhelp
+DATHELP = help hh cmdhelp keyhelp history opthelp optmenu usagehlp wizhelp
 
 SPEC_LEVS = asmodeus.lua baalz.lua bigrm-*.lua castle.lua fakewiz?.lua \
 	juiblex.lua knox.lua medusa-?.lua minend-?.lua minefill.lua \

sys/unix/unixmain.c

@@ -36,6 +36,7 @@ static void consume_arg(int, int *, char ***);
 static void consume_two_args(int, int *, char ***);
 static void early_options(int *, char ***, char **);
 static void opt_terminate(void) NORETURN;
+static void opt_usage(const char *) NORETURN;
 static void opt_showpaths(const char *);
 static void scores_only(int, char **, const char *) NORETURN;
 
@@ -571,6 +572,12 @@ early_options(int *argc_p, char ***argv_p, char **hackdir_p)
 
     config_error_init(FALSE, "command line", FALSE);
 
+    /* treat "nethack ?" as a request for usage info; due to shell
+       processing, player likely has to use "nethack \?" or "nethack '?'"
+       [won't work if used as "nethack -dpath ?" or "nethack -d path ?"] */
+    if (*argc_p > 1 && !strcmp((*argv_p)[1], "?"))
+        opt_usage(*hackdir_p); /* doesn't return */
+
     /*
      * Both *argc_p and *argv_p account for the program name as (*argv_p)[0];
      * local argc and argv impicitly discard that (by starting 'ndx' at 1).
@@ -617,6 +624,12 @@ early_options(int *argc_p, char ***argv_p, char **hackdir_p)
 #endif /* CHDIR */
             }
             break;
+        case 'h':
+        case '?':
+            if (lopt(arg, ArgValDisallowed, "-help", origarg, &argc, &argv)
+                || lopt(arg, ArgValDisallowed, "-?", origarg, &argc, &argv))
+                opt_usage(*hackdir_p); /* doesn't return */
+            break;
         case 'n':
             oldargc = argc;
             if (!strcmp(arg, "-no-nethackrc")) /* no abbreviation allowed */
@@ -653,6 +666,10 @@ early_options(int *argc_p, char ***argv_p, char **hackdir_p)
                 /*NOTREACHED*/
             }
             break;
+        case 'u':
+            if (lopt(arg, ArgValDisallowed, "-usage", origarg, &argc, &argv))
+                opt_usage(*hackdir_p);
+            break;
         case 'v':
             if (argcheck(argc, argv, ARG_VERSION) == 2) {
                 opt_terminate();
@@ -689,6 +706,20 @@ opt_terminate(void)
     /*NOTREACHED*/
 }
 
+static void
+opt_usage(const char *hackdir)
+{
+#ifdef CHDIR
+    chdirx(hackdir, TRUE);
+#else
+    nhUse(hackdir);
+#endif
+    dlb_init();
+
+    genl_display_file(USAGEHELP, TRUE);
+    opt_terminate();
+}
+
 /* show the sysconf file name, playground directory, run-time configuration
    file name, dumplog file name if applicable, and some other things */
 static void