Change ssh.h crypto APIs to output to BinarySink.

This affects all the functions that generate public and private key
and signature blobs of all kinds, plus ssh_ecdhkex_getpublic. Instead
of returning a bare block of memory and taking an extra 'int *length'
parameter, all these functions now write to a BinarySink, and it's the
caller's job to have prepared an appropriate one where they want the
output to go (usually a strbuf).

The main value of this change is that those blob-generation functions
were chock full of ad-hoc length-counting and data marshalling. You
have only to look at rsa2_{public,private}_blob, for example, to see
the kind of thing I was keen to get rid of!

pageant.h

@@ -125,8 +125,7 @@ struct pageant_pubkey {
     /* Everything needed to identify a public key found by
      * pageant_enum_keys and pass it back to the agent or other code
      * later */
-    void *blob;
-    int bloblen;
+    strbuf *blob;
     char *comment;
     int ssh_version;
 };