daemon: Better log when fork child dies early from signals.

daemon: Better log when fork child dies early from signals.

On one machine, "/etc/init.d/openvswitch-switch start" failed to start
with:

   ovs-vswitchd: fork child failed to signal startup (Success)
   Starting ovs-vswitchd ... failed!

"strace" revealed that the fork child was actually segfaulting, but the
message output didn't indicate that in any way.  This commit fixes the
log message (but not the segfault itself).

Reported-by: Michael Hu <mhu@nicira.com>
Bug #8457.