set modified time attribute for tar file.

- for command output, mtime is set to execution time.
- for file, mtime is set to its original mtime.

Currently, unix time is set for both cases, so when you untar, you get warnings like:
```
tar: varnishgather-centox64x86-2013-10-14/023_ps_aux: implausibly old time stamp 1970-01-01 00:00:00
```