Make /dev/kmsg symlinking optional, add lxc.kmsg option

Had this changeset hanging around for some time, maybe this would be useful
until some better solution come up.

Signed-off-by: Alexander Vladimirov <alexander.idkfa.vladimirov@gmail.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
default kmsg symlinking to on, and document lxc.kmsg

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>