various fixes for exporting, including correct handling of errors during export which...

various fixes for exporting, including correct handling of errors during export which previously would hang when using jack1 because jack_set_freewheel() was called from inside a process() callback; use shared_ptr<Route> in parts of export code that weren't using them before; fix up generation of export filenames so that Glib::build_filename() is used and non-existent folders are ignored

git-svn-id: svn://localhost/ardour2/branches/3.0@13610 d708f5d6-7413-0410-9779-e7cbd77b26cf