transmit: Add SIGINT shutdown hook (re)

[Majored]

Hi all (again ;)),

This PR is in relation to #112 and adds:

• A ZST error type representing a user interrupt error so that we can return a non 0/1 exit code.
• The actual SIGINT handling and termination with an exit code of 130.
• Unit tests for terminating an idle transmit (stdin, stdout) and where the transmit is doing actual work.

As was noted in #131 's code comments, there doesn't seem to exist any crate for programmatically sending Ctrl+C events on Windows which I've added a comment about. There's a chance this could be handled via a raw winapi binding but I don't have access to a Windows machine currently to test. I'll look into it in the future if no one else has by then.

[Majored]

Looking at other actions, the Windows failure doesn't seem to be related to anything here.

As for why sigint_termination_idle() is failing, I'm unsure - I can't reproduce the failure on my end (Mac OSX M1). My only guess is that GitHub actions closes the child's stdin & stdout instantly somehow, thus the child exits with a 0 exit code rather than 130 (user interrupt). If so, I'm going to need to change the approach for that test...