Utility function ssh_key_clone().

This makes a second independent copy of an existing ssh_key, for
situations where one piece of code is going to want to keep it after
its current owner frees it.

In order to have it work on an arbitrary ssh_key, whether public-only
or a full public+private key pair, I've had to add an ssh_key query
method to ask whether a private key is known. I'm surprised I haven't
found a need for that before! But I suppose in most situations in an
SSH client you statically know which kind of key you're dealing with.

utils/CMakeLists.txt

@@ -49,6 +49,7 @@ add_sources_from_current_dir(utils
   smemclr.c
   smemeq.c
   spr_get_error_message.c
+  ssh_key_clone.c
   ssh2_pick_fingerprint.c
   sshutils.c
   strbuf.c

utils/ssh_key_clone.c

@@ -0,0 +1,32 @@
+/*
+ * Make a copy of an existing ssh_key object, e.g. to survive after
+ * the original is freed.
+ */
+
+#include "misc.h"
+#include "ssh.h"
+
+ssh_key *ssh_key_clone(ssh_key *key)
+{
+    /*
+     * To avoid having to add a special method in the vtable API, we
+     * clone by round-tripping through public and private blobs.
+     */
+    strbuf *pub = strbuf_new_nm();
+    ssh_key_public_blob(key, BinarySink_UPCAST(pub));
+
+    ssh_key *copy;
+
+    if (ssh_key_has_private(key)) {
+        strbuf *priv = strbuf_new_nm();
+        ssh_key_private_blob(key, BinarySink_UPCAST(priv));
+        copy = ssh_key_new_priv(ssh_key_alg(key), ptrlen_from_strbuf(pub),
+                                ptrlen_from_strbuf(priv));
+        strbuf_free(priv);
+    } else {
+        copy = ssh_key_new_pub(ssh_key_alg(key), ptrlen_from_strbuf(pub));
+    }
+
+    strbuf_free(pub);
+    return copy;
+}