Make termination of Buildbarn binaries a bit more reliable

If a Buildbarn binary is launched as part of a process group and is not
the process group leader, it may be the case that it receives signals
delivered to the process group which are ignored by the process itself.
This causes our final termination signal to never arrive.

Let's put a safety belt in place, calling os.Exit() if a sufficient
amount of time passes.

pkg/program/run_main.go

@@ -7,6 +7,7 @@ import (
 	"os/signal"
 	"sync"
 	"syscall"
+	"time"
 )
 
 // runMainErrorLogger is used by RunMain() to capture errors returned by
@@ -73,12 +74,23 @@ func RunMain(routine Routine) {
 			if err := process.Signal(receivedSignal); err != nil {
 				panic(err)
 			}
+
 			// This code should not be reached, if it
 			// weren't for the fact that process.Signal()
 			// does not guarantee that the signal is
-			// delivered to the same thread. More details:
+			// delivered to the same thread.
+			//
+			// Furthermore, signal.Reset() does not reset
+			// signals that are delivered via the process
+			// group, but ignored by the process itself.
+			// Fall back to calling os.Exit() if we don't
+			// get terminated via signal delivery.
+			//
+			// More details:
 			// https://github.com/golang/go/issues/19326
-			select {}
+			// https://github.com/golang/go/issues/46321
+			time.Sleep(5)
+			os.Exit(1)
 		})
 	}()