imp: forward signals to child's process group in flux-imp run

Problem: `flux-imp run` only forwards signals to its direct child.
When that child is a shell script running other processes, common
signals like SIGINT and SIGTERM are blocked. Even if the direct
child is not a shell, terminating only the IMP's child may leave
stray procesess which do not immediately exit as desired.

Deliver forwarded signals to the child's process group.

Fixes flux-framework#194

src/imp/run.c

@@ -193,7 +193,7 @@ imp_run (struct imp_state *imp,
     if ((child = fork ()) < 0)
         imp_die (1, "run: fork: %s", strerror (errno));
 
-    imp_set_signal_child (child);
+    imp_set_signal_child (-child);
 
     if (child == 0) {
         /* unblock all signals */