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]

be_all.c

@@ -22,10 +22,10 @@ const int be_default_protocol = PROT_TELNET;
 const int be_default_protocol = PROT_SSH;
 #endif
 
-struct backend_list backends[] = {
-    {PROT_SSH, "ssh", &ssh_backend},
-    {PROT_TELNET, "telnet", &telnet_backend},
-    {PROT_RLOGIN, "rlogin", &rlogin_backend},
-    {PROT_RAW, "raw", &raw_backend},
-    {0, NULL}
+Backend *backends[] = {
+    &ssh_backend,
+    &telnet_backend,
+    &rlogin_backend,
+    &raw_backend,
+    NULL
 };