Put proper logging into Pageant.

Now it actually logs all its requests and responses, the fingerprints
of keys mentioned in all messages, and so on.

I've also added the -v option, which causes Pageant in any mode to
direct that logging information to standard error. In --debug mode,
however, the logging output goes to standard output instead (because
when debugging, that information changes from a side effect to the
thing you actually wanted in the first place :-).

An internal tweak: the logging functions now take a va_list rather
than an actual variadic argument list, so that I can pass it through
several functions.

windows/winpgnt.c

@@ -742,7 +742,7 @@ static void answer_msg(void *msgv)
     if (msglen > AGENT_MAX_MSGLEN) {
         reply = pageant_failure_msg(&replylen);
     } else {
-        reply = pageant_handle_msg(msg + 4, msglen, &replylen);
+        reply = pageant_handle_msg(msg + 4, msglen, &replylen, NULL, NULL);
         if (replylen > AGENT_MAX_MSGLEN) {
             smemclr(reply, replylen);
             sfree(reply);