security_dac: Remember old labels

security_dac: Remember old labels

This also requires the same DAC label to be used for shared
paths. If a path is already in use by a domain (or domains) then
and the domain we are starting now wants to access the path it
has to have the same DAC label. This might look too restrictive
as the new label can still guarantee access to already running
domains but in reality it is very unlikely and usually an admin
mistake.

This requirement also simplifies seclabel remembering, because we
can store only one seclabel and have a refcounter for how many
times the path is in use. If we were to allow different labels
and store them in some sort of array the algorithm to match
labels to domains would be needlessly complicated.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>