This also
- move in use options (Opt) from vfsflags to vfscommon
- change os.FileMode to vfscommon.FileMode in parameters
- rework vfscommon.FileMode and add tests
Before this change if we copied files of unknown size, then they lost
their metadata.
This was particularly noticeable using --s3-decompress.
This change adds metadata to Rcat and RcatSized and changes Copy to
pass the metadata in when it calls Rcat for an unknown sized input.
Fixes#6546
