page-xfer: write pidfile from a process which starts a service

When a process, which starts a service, exits, we know that we can
connect to the page server and we expect to find pidfile.

If we create pidfile from a page server process, we have a race window.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>