Upgrade mender client cli

Currently the mender client uses golang "flag" package for parsing commandline arguments. This package is very restrictive and enforcing all flags to have a single hyphen, which is semantically very unintuitive. Moreover, all other services as well as mender-artifact uses `gitlab.com/urfave/cli` to parse commandline arguments.

Acceptance criterion:

• Migrate to using `gitlab.com/urfave/cli` without breaking backwards compatibility with the already existing flags.