Add API for getting all of a key's fingerprints.

ssh2_all_fingerprints() and friends will return a small 'char **'
array, containing all the fingerprints of a key that we know how to
generate, indexed by the FingerprintType enum. The result requires
complex freeing, so there's an ssh2_free_all_fingerprints as well.

For SSH-1 RSA keys, we refuse to generate any fingerprint except the
old SSH-1 MD5 version, because there's no other fingerprint type I
know of that anyone else uses. So I've got a function that returns the
same 'char **' for an SSH-1 key, but it only fills in the MD5 slot,
and leaves the rest NULL.

As a result, I also need a dynamic function that takes a fingerprint
list and returns the id of the most preferred fingerprint type in it
_that actually exists_.

NFC: this API is introduced, but not yet used.

sshpubk.c

@@ -1794,6 +1794,14 @@ char *ssh2_fingerprint_blob(ptrlen blob, FingerprintType fptype)
     return strbuf_to_str(sb);
 }
 
+char **ssh2_all_fingerprints_for_blob(ptrlen blob)
+{
+    char **fps = snewn(SSH_N_FPTYPES, char *);
+    for (unsigned i = 0; i < SSH_N_FPTYPES; i++)
+        fps[i] = ssh2_fingerprint_blob(blob, i);
+    return fps;
+}
+
 char *ssh2_fingerprint(ssh_key *data, FingerprintType fptype)
 {
     strbuf *blob = strbuf_new();
@@ -1803,6 +1811,22 @@ char *ssh2_fingerprint(ssh_key *data, FingerprintType fptype)
     return ret;
 }
 
+char **ssh2_all_fingerprints(ssh_key *data)
+{
+    strbuf *blob = strbuf_new();
+    ssh_key_public_blob(data, BinarySink_UPCAST(blob));
+    char **ret = ssh2_all_fingerprints_for_blob(ptrlen_from_strbuf(blob));
+    strbuf_free(blob);
+    return ret;
+}
+
+void ssh2_free_all_fingerprints(char **fps)
+{
+    for (unsigned i = 0; i < SSH_N_FPTYPES; i++)
+        sfree(fps[i]);
+    sfree(fps);
+}
+
 /* ----------------------------------------------------------------------
  * Determine the type of a private key file.
  */