Moving nm-online to async init introduced various issues:
- with a timeout of zero, nm-online would terminate with failure
  before initializing the NMClient instance.
- add a timeout for safeguarding the async creation of NMClient
- fix adding trailing newline for the progress bar.

While at it, refactor:
- use defines for the exit codes
- don't use exit(), but instead always properly quit the mainloop
  and cleanup all resources.
- in non-quiet mode, print the result [online] or [offline] at
  the end.

Fixes: c5f17a97

https://bugzilla.gnome.org/show_bug.cgi?id=777914