Disable udev fallback by default and add activation/udev_fallback to lvm.conf.

Disable udev fallback by default and add activation/udev_fallback to lvm.conf.

We've used udev fallback code till now to check whether udev
created/removed the entries in /dev correctly and if not,
a repair was done (giving a warning messagea about that).

This patch adds a possibility to enable this additional check
and subsequent fallback only when required (debugging purposes
mostly) and trust udev completely.

So let's disable the fallback code by default and add a new
configuration option "activation/udev_fallback".

(The original code for creating the nodes will still be used
in case the device directory that is set in lvm.conf differs
from the one that udev uses and also when activation/udev_rules
is set to 0 - otherwise we would end up with no nodes/symlinks
at all)