Client disconnect on SIGINT

[lcsmuller]

What?

Disconnect client(s) from their on-going session if a SIGINT is received.

Why?

• Abruptly ending the program means Discord would never be notified of the client's disconnect, thus keeping a lingering online status until it fails to receive a heartbeat payload.
• Ensure full cleanup of the API resources so that users may catch up their memory leaks, or incorrect cleanup from concord's side.

How?

A ccord_has_sigint global is set once a SIGINT is detected, which will be checked against by all running clients and they will be prompted to call discord_shutdown().

Testing?

Removed now outdated test/async.c logic that had a custom SIGINT handler for gracefully disconnecting, with this PR Ctrl+C for a clean disconnect still works just fine.

Anything Else? (optional)

I haven't added any mutexes around the ccord_has_sigint global var, doing so is trivial but I couldn't think of a scenario where multiple SIGINT may be triggered in parallel.