compel: Improve infect

 - extend handle_sigchld
 - fix garbage return in compel_prepare
 - handle errors in make_sock_for

travis-ci: success for compel: A few fixes and example
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>