* fix: separate signals for passive, active, and forced shutdown
`SIGTERM`: Passive shutdown stopping provisioner daemons from accepting new
jobs but waiting for existing jobs to successfully complete.
`SIGINT` (old existing behavior): Notify provisioner daemons to cancel in-flight jobs, wait 5s for jobs to be exited, then force quit.
`SIGKILL`: Untouched from before, will force-quit.
* Revert dramatic signal changes
* Rename
* Fix shutdown behavior for provisioner daemons
* Add test for graceful shutdown
Fixes an issue where remote forwards are not correctly torn down when using OpenSSH with `coder ssh --stdio`. OpenSSH sends a disconnect signal, but then also sends SIGHUP to `coder`. Previously, we just exited when we got SIGHUP, and this raced against properly disconnecting.
Fixes https://github.com/coder/customers/issues/327
Steven Masley
Ammar Bandukwala
Kyle Carberry
Colin Adler
Spike Curtis