Support systemd watchdog.

### Checklist

- [x] My issue is specific & actionable.
- [x] I am not suggesting a protocol enhancement.
- [x] I have searched on the issue tracker for my issue.

### Description

In my experience go-ipfs hangs quite frequently. This is of course a bug that should be addressed but bugs such as this will always appear again. In order to help mitigate the hanging issue go-ipfs should support systemd's watchdog mechanism. This ensures that unless the program notifies the service manager that it is in a working state periodically an action can be taken (such as restarting the service).

The docs for systemd are here: https://www.freedesktop.org/software/systemd/man/sd_notify.html It is simply writing some strings to a socket and is trivial to implement. The hardest part will be ensuring that IPFS is working properly before writing these strings (to ensure that it isn't *only* the watchdog loop that is working correctly).