csync: Keep the csync_file_stat_t instances created during discovery

csync: Keep the csync_file_stat_t instances created during discovery

Now that they use the same structure, avoid _csync_detect_update
having to recreate another instance and transfer everything manually.
Any instance created during discovery should now be used all the way
up to SyncEngine::treewalkFile.

This also makes sure that the path and types are properly set in that
object instead of having to pass everything as separate parameters.

This gets rid of csync_ftw_flags_e which was now converted from,
and to csync_ftw_type_e, already in the csync_file_stat_t.

Issue #1817