Windows Plink: accept a Unicode remote command line.

Just as with other recent changes like usernames, this allows the
remote command line to include characters outside the system code
page, encoding as UTF-8 on the wire (as the SSH protocol has wanted
all along).

windows/plink.c

@@ -375,7 +375,7 @@ int main(int argc, char **argv)
             while (arg) {
                 if (cmdbuf->len > 0)
                     put_byte(cmdbuf, ' '); /* add space separator */
-                put_dataz(cmdbuf, cmdline_arg_to_str(arg));
+                put_dataz(cmdbuf, cmdline_arg_to_utf8(arg));
                 arg = arglist->args[arglistpos++];
             }