fix: gracefully shutdown on CTRL+C and SIGTERM on Linux

[internationalcrisis]

When running on Linux, listen for SIGTERM in addition to CTRL+C.

I am not incredibly competent with Rust and Linux is my daily driver, so please double-check my changes. Regardless, this should behave relatively the same on Linux and Windows, past the "Failed to install CTRL+C signal handler" error not appearing on Linux if redlib fails to install that signal handler.

This also fixes #205.