This adds an additional parameter to the creation of each flag. This
specifies one or more flag groups. This **must** be set for global
flags and **must not** be set for local flags.
This causes flags.md to be built with sections to aid comprehension
and it causes the documentation pages for each command (and the
`--help`) to be built showing the flags groups as specified in the
`groups` annotation on the command.
See: https://forum.rclone.org/t/make-docs-for-mortals-not-only-rclone-gurus/39476/
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.
