config.c: loop over backends list for protocol selector.

Similarly to the previous commit, this is one fewer place where I need
to make a handwritten change with each new protocol.

putty.h

@@ -368,7 +368,8 @@ enum {
     PROT_RAW, PROT_TELNET, PROT_RLOGIN, PROT_SSH,
     /* PROT_SERIAL is supported on a subset of platforms, but it doesn't
      * hurt to define it globally. */
-    PROT_SERIAL
+    PROT_SERIAL,
+    PROTOCOL_LIMIT, /* upper bound on number of protocols */
 };
 
 enum {