Return zero when reporting our version.

When called with -V to ask for our version, return 0 rather than 1.
This is the usual behaviour observed by ssh(1) and other Unix commands.
Also use exit() rather than cleanup_exit() in pscp.c and psftp.c ; at
this point we have nothing to cleanup!

windows/winplink.c

@@ -228,7 +228,7 @@ static void version(void)
     char *buildinfo_text = buildinfo("\n");
     printf("plink: %s\n%s\n", ver, buildinfo_text);
     sfree(buildinfo_text);
-    exit(1);
+    exit(0);
 }
 
 char *do_select(SOCKET skt, int startup)