Allow command inputs to be overwritten safely
Allow command inputs to be overwritten safely

Addresses #842. Turns out it's a simple matter of deleting the file
rather than opening with O_TRUNC. If the file is already opened, the
kernel ensures it remains usable by processes that hold its file
descriptor.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3