Gracefully close miner application

[mvastarelli]

When stopping the miner application, it is terminated by sending the SIGKILL to all processes with the same name. This is problematic for two reasons.

1. If there are other miners running on the system (there shouldn't, but it's not impossible) they will be terminated as well.
2. Any GPU-specific settings (e.g. locking GPU core clock) will not be reset.

To fix we need to look into other ways of halting the process cleanly. For example, we should look into hooking into STDIN and sending CTRL+C to the process window instead.