v3 service: SIGCHLD handler

Hi!

Now we're ignoring SIGCHLD from forked children.
This patch provides SIGCHLD handler, that will wait for child and write some
information about it's exit status to logfile.

Signed-off-by: Ruslan Kuprieiev <kupruser@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>