Make restorer-log.c to be more similar to general log code.

 - Use current_logfd/current_loglevel variable names
 - Add ability to filter out messages which log level
   is not requested

For example, if a program get restorer without any log
level specified -- we restore it silently not printing
even a single message (if no error happened of course).
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>