Friendlier archive names, tarsnap error handling

1) When using paths like /path/to/my/folder inside tarsnap.folders, tarsnap-generations will use that as $dir in the filename, which can cause issues because of slashes. Proposed method is to replace slashes with dots to keep filenames harmless.

2)Line 124, the exit $? exits with the status of the last echo, which is always 0. Better to store tarsnap's code and use it in that exit statement.