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.
Add --network-mode option to activate mounting as network drive without having to set volume prefix.
Add support for automatic drive letter assignment (not specific to network drive mounting).
Allow full network share unc path in --volname, which will also implicitely activate network drive mounting.
Allow full network share unc path as mountpoint, which will also implicitely activate network drive mounting, and the specified path will be used as volume prefix and the remote will be mounted on an automatically assigned drive letter instead.
