Ignore SIGINT in the Rust worker.

[peterhuene]

This commit ignores SIGINT on both POSIX and Windows, preventing the worker
process from terminating before the Azure Functions Host can gracefully
shutdown the worker process.

However, the Azure Function Host doesn't yet gracefully terminate the worker
process. Without this fix, though, the worker won't properly shut down once
the host does gracefully terminate the worker.

See Azure/azure-functions-host#3296 for that issue.

Fixes #153.

[peterhuene]

@rylev as you're on vacation and this is such a small change, I'm going to merge this and release 0.4.1 with the recent fixes.

Also, I've investigated what it would take for a cargo func deploy command to use a combination of the Azure CLI and Azure Function Core Tools to create the functions app (if necessary) and configure it for use with a deployed image. Let's discuss next week.