Rationalise access to, and content of, backends[] array.

Should be no significant change in behaviour.
(Well, entering usernames containing commas on Plink's command line will be
a little harder now.)

[originally from svn r7628]

unix/uxpty.c

@@ -1085,5 +1085,7 @@ Backend pty_backend = {
     pty_provide_logctx,
     pty_unthrottle,
     pty_cfg_info,
+    "pty",
+    -1,
     1
 };