log: Add pr_quelled helper

If we need to check if current loglevel will suppress
our messagess (say you need to run pr_debug in a cycle)
we can use this helper to eliminate unneded calls.

Like
  if (!pr_quelled(LOG_DEBUG)) {
    ... do something specific to LOG_DEBUG ...
  }

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