Properly handle interrupt signals

[felddy]

🗣 Description

This PR fixes an issue with signal handling within interactive shells of child processes. Specifically,
pressing control-c in a shell causes awssh to exit instead of allowing the shell to interpret the signal.

💭 Motivation and context

Currently, control-c will exit awssh which ends a user's remote session.
This is broken and infuriating. By setting an ignore action on the interrupt signal, in the parent,
before thesubprocess call, the child can then handle the signal as expected.
The signal's action in the parent is restored after the child exits for good measure.

🧪 Testing

Tested on my local machine by connecting to various AWS instances and furiously mashing control-c.
Tested with standard suite of continuous integration actions.

✅ Pre-approval checklist

• This PR has an informative and human-readable title.
• Changes are limited to a single goal - eschew scope creep!
• All relevant type-of-change labels have been added.
• I have read the CONTRIBUTING document.
• These code changes follow cisagov code standards.
• All new and existing tests pass.

✅ Post-merge checklist

• Add a tag or create a release.

[mcdonnnj]

[dav3r]

Nice and tidy. 👍
Thanks for looking into this and fixing it!