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.
