Don't print long usage messages on a command-line error.

In the course of debugging the command-line argument refactoring in
previous commits, I found I wasn't quite sure whether PSCP thought I'd
given it too many arguments, or too few, because it didn't print an
error message saying which: it just printed its giant usage message.

Over the last few years I've come to the belief that this is Just
Wrong anyway. Printing the whole of a giant help message should only
be done when the user asked for it: otherwise, print a short and
to-the-point error, and maybe _suggest_ how to get help, but scrolling
everything else off the user's screen is not a good response to a
typo. I wrote this thought up more fully last year:

https://www.chiark.greenend.org.uk/~sgtatham/quasiblog/stop-helping/

So, time to practise what I preach! The PuTTY tools now follow the
'Stop helping!' principle. You can get full help by saying --help.

Also, when we do print the help, we now exit(0) rather than exit(1),
because there's no reason to report failure: we successfully did what
the user asked us for.

psftp.c

@@ -2565,7 +2565,6 @@ static void usage(void)
     printf("  -logoverwrite\n");
     printf("  -logappend\n");
     printf("            control what happens when a log file already exists\n");
-    cleanup_exit(1);
 }
 
 static void version(void)
@@ -2819,7 +2818,7 @@ int psftp_main(CmdlineArgList *arglist)
 
         if (argstr[0] != '-') {
             if (userhost)
-                usage();
+                cmdline_error("unexpected extra argument \"%s\"", argstr);
             else
                 userhost = dupstr(argstr);
             continue;
@@ -2837,6 +2836,7 @@ int psftp_main(CmdlineArgList *arglist)
                    strcmp(argstr, "-?") == 0 ||
                    strcmp(argstr, "--help") == 0) {
             usage();
+            cleanup_exit(0);
         } else if (strcmp(argstr, "-pgpfp") == 0) {
             pgp_fingerprints();
             return 1;