apparmor: revalidate files during exec

apparmor: revalidate files during exec

Instead of running file revalidation lazily when read/write are called
copy selinux and revalidate the file table on exec. This avoids
extra mediation overhead in read/write and also prevents file handles
being passed through to a grand child unchecked.

Signed-off-by: John Johansen <john.johansen@canonical.com>