fix: handle SIGHUP from OpenSSH (#10638)

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

tailnet/conn.go

@@ -788,6 +788,7 @@ func (c *Conn) Close() error {
 	}
 
 	_ = c.netStack.Close()
+	c.logger.Debug(context.Background(), "closed netstack")
 	c.dialCancel()
 	_ = c.wireguardMonitor.Close()
 	_ = c.dialer.Close()