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_none.c

@@ -1,16 +1,11 @@
 /*
- * Linking module for PSCP: list the available backends, but
- * without accompanying function suites. Used only for name
- * lookups.
+ * Linking module for programs that do not support selection of backend
+ * (such as pscp or pterm).
  */
 
 #include <stdio.h>
 #include "putty.h"
 
-struct backend_list backends[] = {
-    {PROT_SSH, "ssh", NULL},
-    {PROT_TELNET, "telnet", NULL},
-    {PROT_RLOGIN, "rlogin", NULL},
-    {PROT_RAW, "raw", NULL},
-    {0, NULL}
+Backend *backends[] = {
+    NULL
 };