apparmor: make str table more generic and be able to have multiple entries

apparmor: make str table more generic and be able to have multiple entries

The strtable is currently limited to a single entry string on unpack
even though domain has the concept of multiple entries within it. Make
this a reality as it will be used for tags and more advanced domain
transitions.

Reviewed-by: Georgia Garcia <georgia.garcia@canonical.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>