Be sure to initialise ExportFormatSpecification::_soundcloud_upload

Initialise _soundcloud_upload in all constructors of
ExportFormatSpecification.
Fix spurious 'You cannot put a CD marker at this location' error

Fix the spurious error that occurs when loading a session where any marker
(not necessarily a CD marker) is located at 0 on the timeline.
Make zoom-to-session include regions currently being recorded
Return playhead to last start position when aborting capture
Fix restore of Jack driver setting

A small hack to ensure that the driver setting in the audio engine dialog
is restored when ardour starts up and when switching drivers.