Pageant core: initial deferred decryption facility.

This adds an extension request to the agent protocol (named in our
private namespace, naturally) which allows you to upload a key file in
the form of a string containing an entire .ppk file. If the key is
encrypted, then Pageant stores it in such a way that it will show up
in the key list, and on the first attempt to sign something with it,
prompt for a passphrase (if it can), decrypt the key, and then answer
the request.

There are a lot of rough edges still to deal with, but this is good
enough to have successfully answered one request, so it's a start.

ssh.h

@@ -1497,6 +1497,7 @@ enum {
 #define SSH2_AGENTC_ADD_IDENTITY                17
 #define SSH2_AGENTC_REMOVE_IDENTITY             18
 #define SSH2_AGENTC_REMOVE_ALL_IDENTITIES       19
+#define SSH2_AGENTC_EXTENSION                   27
 
 /*
  * Assorted other SSH-related enumerations.