Command line configuration overrides.

For review, a --set command line option for setting arbitrary configuration values.  Example usage:

Turn on the unix-command option from the command line:
```
--set unix-command.enabled=yes
```

Change the thread count for the first specified af-packet interface:
```
--set af-packet.0.threads=6
```

Change the default detect-thread-ratio from the command line:
```
--set threading.detect-thread-ration=1.5
```

I'm indifferent on the argument being "--set" if anyone thinks there is a better name for it.

Buildbot:
https://buildbot.suricata-ids.org/builders/jasonish/builds/48
pcap-buildbot was failing to accept a build.