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.