Rework the Pageant client code to use BinarySource.

There was a lot of ugly, repetitive, error-prone code that decoded
agent responses in raw data buffers. Now my internal client query
function is returning something that works as a BinarySource, so we
can decode agent responses using the marshal.h system like any other
SSH-formatted message in this code base.

While I'm at it, I've centralised more of the parsing of key lists
(saving repetition in pageant_add_key and pageant_enum_keys),
including merging most of the logic between SSH-1 and SSH-2. The old
functions pageant_get_keylist1 and pageant_get_keylist2 aren't exposed
in pageant.h any more, because they no longer exist in that form, and
also because nothing was using them anyway. (Windows Pageant was using
the separate pageant_nth_ssh2_key() functions that talk directly to
the core, and Unix Pageant was using the more cooked client function
pageant_enum_keys.)

pageant.h

@@ -194,10 +194,6 @@ void pageant_listener_free(struct pageant_listen_state *pl);
  * process. (On at least one platform we want to do this in an
  * agnostic way between the two situations.)
  *
- * pageant_get_keylist{1,2} work just like pageant_make_keylist{1,2}
- * above, except that they can also cope if they have to contact an
- * external agent.
- *
  * pageant_add_keyfile() is used to load a private key from a file and
  * add it to the agent. Initially, you should call it with passphrase
  * NULL, and it will check if the key is already in the agent, and
@@ -212,8 +208,6 @@ void pageant_listener_free(struct pageant_listen_state *pl);
  * for keys that have the same trust properties). Call
  * pageant_forget_passphrases() to get rid of them all.
  */
-void *pageant_get_keylist1(int *length);
-void *pageant_get_keylist2(int *length);
 enum {
     PAGEANT_ACTION_OK,       /* success; no further action needed */
     PAGEANT_ACTION_FAILURE,  /* failure; *retstr is error message */