userauth: derive publickey from private

Pass a NULL pointer for the publickey parameter of
libssh2_userauth_publickey_fromfile and
libssh2_userauth_hostbased_fromfile functions.  In this case, the
functions recompute the public key from the private key file data.

This is work done by Jean-Louis CHARTON
<Jean-Louis.CHARTON@oikialog.com>, then adapted by Mark Smith and
slightly edited further by me Daniel.

WARNING: this does leave the feature NOT WORKING when libssh2 is built
to use libgcrypt instead of OpenSSL simply due to lack of
implementation.

src/libgcrypt.c

@@ -572,4 +572,17 @@ _libssh2_cipher_crypt(_libssh2_cipher_ctx * ctx,
     return ret;
 }
 
+int
+_libssh2_pub_priv_keyfile(LIBSSH2_SESSION *session,
+                          unsigned char **method,
+                          size_t *method_len,
+                          unsigned char **pubkeydata,
+                          size_t *pubkeydata_len,
+                          const char *privatekey,
+                          const char *passphrase)
+{
+    return -1; /* not yet supported; interpreted by userauth.c to call
+                  libssh2_error */
+}
+
 #endif /* LIBSSH2_LIBGCRYPT */