By doing graceful shutdown, we're able to properly close all of the open
resources such as modem connections and ongoing sessions.
This will allow transport drivers controlling stateful TNCs (e.g.
ptc-go) to recover the initial TNC state before termination.
Resolves#314
* Replace(..., -1) with ReplaceAll
* Variable names hiding packages
* Error messages start with lowercase and no end punctuation
* Don't `defer` in a loop
* Etc.
This new command style hopefully makes usage easier and
clearer. Usage of connect has been improved.
It is no longer possible to run http server while in
interactive mode.
