criu: change the way -v works

For most of the CLI tools I see, adding -v option increases their verbosity.

Currently, this is not the case for criu. Default verbosity is set it 2,
and using -v actually decreases the verbosity level to 1, so to increase
verbosity you need to use -vvv (or -v3). To me, this behavior is
quite counterintuitive.

This patch changes the way -v works (but retains -vNUM as it was). Now,
using -v increases verbosity by +1 from the default value (and e.g. -vvv
increases it by +3).

Surely, this changes user experience: for example someone who
was using -v (rather than -v1 or -v0) to silent warnings, will
now have verbosity increased. IMHO this is for the better.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>