Move gateway selection helpers to backend.
Handle trying to kill a non running process.
Remove signaling logs, use err for unknown signal.
Use less confusing signal names.

Renames:
eip_download_config -> eip_config_ready
eip_download_client_certificate -> eip_client_certificate_ready
Improve backend docstrings.

* Update wording to match pep-0257
* Add docstrings for signals emitted by the Backend.
Remove unneeded parent's constructor call.