Pageant: handle agent extension messages more correctly.

Reading draft-miller-ssh-agent-04 more carefully, I see that I missed
a few things from the extension-message spec. Firstly, there's an
extension request "query" which is supposed to list all the extensions
you support. Secondly, if you recognise an extension-request name but
are then unable to fulfill the request for some other reason, you're
supposed to return a new kind of failure message that's distinct from
SSH_AGENT_FAILURE, because for extensions, the latter is reserved for
"I don't even know what this extension name means at all".

I've fixed both of those bugs in Pageant by making a centralised map
of known extension names to an enumeration of internal ids, and an
array containing the name for each id. So we can reliably answer the
"query" extension by iterating over that array, and also use the same
array to recognise known extensions up front and give them centralised
processing (in particular, resetting the failure-message type) before
switching on the particular extension index.

ssh.h

@@ -1491,6 +1491,7 @@ enum {
 #define SSH2_AGENTC_REMOVE_IDENTITY             18
 #define SSH2_AGENTC_REMOVE_ALL_IDENTITIES       19
 #define SSH2_AGENTC_EXTENSION                   27
+#define SSH_AGENT_EXTENSION_FAILURE             28
 
 /*
  * Assorted other SSH-related enumerations.