transmit: Add SIGINT shutdown hook

[Majored]

Hi all,

This PR is in relation to: #112

A pretty simple addition; we periodically (every 500ms) check if the SIGINT flag has been set and break out of the piping loop if so (and thus the sinks are shutdown/closed).
Also slightly refactors the loop to be a bit more readable in the select! macro context.

[russelltg]

Hello, welcome and why'd you close this immediately? Fyi: #131 has a start to this functionality, and has a unit test that may be functional--I don't remember exactly. My bad for not having my PR linked to that issue.

However, that branch is stale and you're still welcome to take this issue!

[Majored]

I hadn't seen #131 originally, so closed this and was going to comment on it later to see what your intentions were (but didn't expect you to reply this quickly ;)).

Are you happy with the approach used? If so, I'll be happy to take a look at your unit test and incorporate it (along with looking into one for Windows given your comment).