imp: run: put child in new process group

Problem: Before the removal of `flux-imp kill`, commands invoked by
`flux-imp run` were signaled by process group instead of an individual
process ID. This worked well because `flux-imp run` replaced itself
with the target command, and thus the command and its children were
in a process group (set up by flux-core's libsubprocess). Now, the
IMP itself will be in that process group, and if it tries to forward
signals to the entire process group it will end up re-signaling itself.

Call `setpgrp(2)` in the child to place it in its own process group.
The IMP now has a process group to which it can forward signals.

src/imp/run.c

@@ -199,6 +199,12 @@ imp_run (struct imp_state *imp,
         /* unblock all signals */
         imp_sigunblock_all ();
 
+        /* Place child in its own process group, so that parent IMP
+         * can signal the pgrp as a whole
+         */
+        if (setpgrp () < 0)
+            imp_die (1, "setpgrp: %s", strerror (errno));
+
         if (setuid (geteuid()) < 0
             || setgid (getegid()) < 0)
             imp_die (1, "setuid: %s", strerror (errno));