POSIX Signal Handler and Proxy/Bus

[krisnova]

Now that auraed is launching nested versions of itself, we will need to proxy POSIX signals through auraed

1. Implement a POSIX compliant signal handler in Rust in Auraed and provide documentation on which signals map to what behavior.

SIGKILL should terminate (kill) the process SIGHUP should reload the config from disk and reopen logfiles SIGINT should "interrupt" the process and begin to "die nice" ensuring any cleanup logic can be done

2. Use SIGINT instead of SIGKILL to "free" a nested auraed after the signal handler has been implemented.

3. Proxy all signals to nested executables for them to manage independently.

For example sending a SIGHUP to a nested auraed should proxy a SIGHUP to all of the nested executables!