Globals are bad. Don't let nmc_readline_helper() access
nm_cli.

Instead, pass nmc_config along. nmc_config albeit being
a complex struct, is much more begning:

  - the configuration nmc_config is initialized early on
    and afterwards immutable.
  - it only contains simple fields, which affect the behavior.
  - it's not a global. While passing around the complex configuration
    struct, it is clear that all callpaths don't access additional
    global information.