sftp: force openpty(3) on BSD

Implementation of grantpt() and unlockpt() on FreeBSD and OpenBSD does
not conform to IEEE Std 1003.1-2008 (``POSIX.1'') so force using the
openpty(3) code path so that the key fingerprint and login/password
dialogs instead of just failing or falling back to SSH_ASKPASS.

https://bugzilla.gnome.org/show_bug.cgi?id=722001