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.

cmdgen.c

@@ -718,10 +718,11 @@ int main(int argc, char **argv)
 
     /*
      * If run with at least one argument _but_ not the required
-     * ones, print the usage message and return failure.
+     * ones, fail with an error.
      */
     if (!infile && keytype == NOKEYGEN) {
-        usage(true);
+        fprintf(stderr, "puttygen: expected an input key file name, "
+                "or -t for a type of key to generate\n");
         RETURN(1);
     }
 

pscp.c

@@ -2236,7 +2236,6 @@ static void usage(void)
     printf("  -logoverwrite\n");
     printf("  -logappend\n");
     printf("            control what happens when a log file already exists\n");
-    cleanup_exit(1);
 }
 
 void version(void)
@@ -2311,6 +2310,7 @@ int psftp_main(CmdlineArgList *arglist)
                    strcmp(argstr, "-?") == 0 ||
                    strcmp(argstr, "--help") == 0) {
             usage();
+            cleanup_exit(0);
         } else if (strcmp(argstr, "-V") == 0 ||
                    strcmp(argstr, "--version") == 0) {
             version();
@@ -2351,12 +2351,12 @@ int psftp_main(CmdlineArgList *arglist)
 
     if (list) {
         if (nscpargs != 1)
-            usage();
+            cmdline_error("expected a single argument with -ls");
         get_dir_list(scpargs, nscpargs);
 
     } else {
         if (nscpargs < 2)
-            usage();
+            cmdline_error("expected at least two arguments");
         if (nscpargs > 2)
             targetshouldbedirectory = true;
 

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;

unix/pageant.c

@@ -218,7 +218,6 @@ static void usage(void)
     printf("  --tty-prompt force tty-based passphrase prompt\n");
     printf("  --gui-prompt force GUI-based passphrase prompt\n");
     printf("  --askpass <prompt>   behave like a standalone askpass program\n");
-    exit(1);
 }
 
 static void version(void)

unix/plink.c

@@ -573,7 +573,6 @@ static void usage(void)
     printf("            control what happens when a log file already exists\n");
     printf("  -shareexists\n");
     printf("            test whether a connection-sharing upstream exists\n");
-    exit(1);
 }
 
 static void version(void)
@@ -807,7 +806,10 @@ int main(int argc, char **argv)
         return 1;
 
     if (!cmdline_host_ok(conf)) {
-        usage();
+        fprintf(stderr, "plink: no valid host name provided\n"
+                "try \"plink --help\" for help\n");
+        cmdline_arg_list_free(arglist);
+        return 1;
     }
 
     prepare_session(conf);

windows/plink.c

@@ -189,7 +189,6 @@ static void usage(void)
     printf("            control what happens when a log file already exists\n");
     printf("  -shareexists\n");
     printf("            test whether a connection-sharing upstream exists\n");
-    exit(1);
 }
 
 static void version(void)
@@ -350,6 +349,7 @@ int main(int argc, char **argv)
             version();
         } else if (!strcmp(p, "--help")) {
             usage();
+            exit(0);
         } else if (!strcmp(p, "-pgpfp")) {
             pgp_fingerprints();
             exit(1);
@@ -395,7 +395,10 @@ int main(int argc, char **argv)
         return 1;
 
     if (!cmdline_host_ok(conf)) {
-        usage();
+        fprintf(stderr, "plink: no valid host name provided\n"
+                "try \"plink --help\" for help\n");
+        cmdline_arg_list_free(arglist);
+        return 1;
     }
 
     prepare_session(conf);