Only make new process a group leader, not session

We're not dealing with a debugged process having its own controlling
terminal at this point, so no need to make the new process a session
leader. Simply making the process a group leader will suffice for our
purposes at the moment.

proc/exec_darwin.c

@@ -37,13 +37,9 @@ fork_exec(char *argv0, char **argv, int size,
 	read(fd[0], &sig, 1);
 	close(fd[0]);
 
-	// Create a new session for this process.
-	if (setsid() < 0) {
-		// Could not set session, but still want to create
-		// a new process group.
-		if (setpgid(0, 0) < 0) {
-			return -1;
-		}
+	// Create a new process group.
+	if (setpgid(0, 0) < 0) {
+		return -1;
 	}
 
 	// Set errno to zero before a call to ptrace.

proc/proc_linux.go

@@ -44,7 +44,7 @@ func Launch(cmd []string) (*Process, error) {
 		proc.Args = cmd
 		proc.Stdout = os.Stdout
 		proc.Stderr = os.Stderr
-		proc.SysProcAttr = &syscall.SysProcAttr{Ptrace: true, Setsid: true}
+		proc.SysProcAttr = &syscall.SysProcAttr{Ptrace: true, Setpgid: true}
 		err = proc.Start()
 	})
 	if err != nil {