Due to a logic error files stored on remotes which support modtime but
not hashes weren't being transferred when updating with a file of the
same size but different modtime. Instead the modtime of the remote
file was being set to that of the local file.
In practice this affected crypt with all remotes except Amazon Drive
and Dropbox.
* Make move command check for overlapping remotes and refuse to run
* Do copy/delete rather than all the copies then all the deletes
* Doesn't purge the source - this was unexpected behaviour see #512 and #416
* Add -list-retries flag to test suite to control retries
This changes the semantics of `move` slightly. However it now errs on
the side of not deleting stuff.
* Factor sync/copy/move into its own file
* Make fatal errors abort the sync
* Make Copy return errors
* Make Sync/Copy/Move return the last Copy error if there was one
* Prioritise returning Fatal errors
* NoRetry errors are returned if no other types of errors
