apparmor: ensure that undecidable profile attachments fail

Profiles that have an undecidable overlap in their attachments are
being incorrectly handled. Instead of failing to attach the first one
encountered is being used.

eg.
  profile A /** { .. }
  profile B /*foo { .. }

have an unresolvable longest left attachment, they both have an exact
match on / and then have an overlapping expression that has no clear
winner.

Currently the winner will be the profile that is loaded first which
can result in non-deterministic behavior. Instead in this situation
the exec should fail.

Fixes: 898127c3

 ("AppArmor: functions for domain transitions")
Signed-off-by: John Johansen <john.johansen@canonical.com>