fix up enum mess caused by switching from JACK to Engine as the name for jack-related transport sync

This will allow older versions of Ardour to continue to startup when the user sync preference is "JACK",
because the string used by enum_2_string() will still be "JACK".

Versions of ardour3 from git after the enum change until this commit will leave ardour.rc unloaded by old
versions of Ardour ***if*** the user sync choice was "JACK".