apparmor: document first entry is in packed perms struct is reserved

Add a comment to unpack_perm to document the first entry in the packed
perms struct is reserved, and make a non-functional change of unpacking
to a temporary stack variable named "reserved" to help suppor the
documentation of which value is reserved.

Suggested-by: Serge E. Hallyn <serge@hallyn.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>