Unix Pageant: provide public-key extraction options.

I've decided against implementing an option exactly analogous to
'ssh-add -L' (printing the full public key of everything in the
agent). Instead, you can identify a specific key to display in full,
by any of the same means -d lets you use, and then print it in either
of the public key formats we support.

pageant.h

@@ -127,6 +127,7 @@ struct pageant_pubkey {
      * later */
     void *blob;
     int bloblen;
+    char *comment;
     int ssh_version;
 };
 struct pageant_pubkey *pageant_pubkey_copy(struct pageant_pubkey *key);