Don't use standard descriptors for logging

The standard descriptors may be redirected.

crtool dumplicates stderr in rlimit.maxfileno-1 and this descriptor
is inherited by all children and will be closed before sigreturn.

Known issues:

 - The logging descriptor may be used by a target process and
   a resume will fail.

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