Check for OpenSSH and implement sftp_symlink correct.

When OpenSSH's sftp-server was implemented, the order of the arguments
to the SSH_FXP_SYMLINK method was inadvertently reversed. Unfortunately,
the reversal was not noticed until the server was widely deployed.
Since fixing this to follow the specification would cause
incompatibility, the current order was retained.

include/libssh/libssh.h

@@ -217,6 +217,7 @@ int ssh_connect(SSH_SESSION *session);
 void ssh_disconnect(SSH_SESSION *session);
 int ssh_service_request(SSH_SESSION *session, const char *service);
 char *ssh_get_issue_banner(SSH_SESSION *session);
+int ssh_get_openssh_version(ssh_session session);
 /* get copyright informations */
 const char *ssh_copyright(void);