Before this change, this code implemented an ad-hoc parser for a
subset of vfs and mount options.
After the config re-organization it can use the same parsing code as
the rest of rclone which simplifies the code and exposes all the VFS
and mount options.
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
This is possible now that we no longer support go1.12 and brings
rclone into line with standard practices in the Go world.
This also removes errors.New and errors.Errorf from lib/errors and
prefers the stdlib errors package over lib/errors.