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]

raw.c

@@ -278,5 +278,7 @@ Backend raw_backend = {
     raw_provide_logctx,
     raw_unthrottle,
     raw_cfg_info,
+    "raw",
+    PROT_RAW,
     1
 };