FU: Log unclean worker shutdowns in supervisor

This is mainly to monitor kills from OOM or other situations that the
worker process itself is unable to log properly.

FU.pm

@@ -491,6 +491,8 @@ sub _supervisor($c) {
             if (!$err && (!$childs{$pid} || $childs{$pid} != 2)) {
                 $err = 1;
                 log_write "Script exited before calling FU::run()\n";
+            } elsif ($?) {
+                log_write "Unclean shutdown of worker PID $pid status $?\n";
             }
             delete $childs{$pid};
         }