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.
Before this patch selfupdate detected ANY build with cmount tag as a build
having libFUSE capabilities. However, only dynamic builds really have it.
The official linux builds are static and have the cmount tag as of the time
of this writing. This results in inability to update official linux binaries.
This patch fixes that. The build can be fixed independently.
