This removes all of the global loggers from the project and replaces them with loggers which are instantiated in vspd.go and passed down as params.
To support this, the background package was removed. It only contained one file so it was a bit pointless anyway. It only existed so background tasks could have their own named logger.
Bring in some updates from the signal handling code in other projects:
- Rename `signals` to more descriptive `interruptSignals`.
- Add SIGHUP to unix shutdown signals. Rename `signalsigterm.go` to `signal_unix.go` accordingly.
- Include extra detail in "Already shutting down..." messages log lines.
- Pass a shutdown func into `webapi.go` rather than a channel. It's more obvious how to invoke a func, whereas a channel can be used in multiple ways.
