fixed: restore w/ overwrite by linux client and split vss

###### Setup:
``win-client`` is a windows client configured with ``split_vss=1`` 
``linux-client`` in a linux client configured as ``restore_client`` for ``win-client``.

###### Problem:
When running the following on ``linux-client`` (note the ``-f`` option)

     burp -C win-client -a r -f -d <restore path> -b <#backup> -r <regexp>

``burp`` first overwrites any existing file at the destination with restored file data and then overwrites that with the vss footer of each file - end result is that we're left with just the vss footer.

###### Fix:
on linux: prevent overwrite of file when considering vss footer in ``overwrite_ok``

Please review and pull.
Thanks,
Avi