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.
This is implemented as a state machine parser so it can emit sensible
error messages.
It does not use the connection strings elsewhere in rclone yet - see
subsequent commits.
An optional fuzzer is implemented for the Parse function.
Currently if container under test has multiple IP addresses,
the `docker_ip` function from `docker.sh` will return a gibberish.
This patch makes it return the first address found.
Additionally, I apply shellcheck on `docker.sh`.
