apparmor: fix x_table_lookup when stacking is not the first entry

x_table_lookup currently does stacking during label_parse() if the
target specifies a stack but its only caller ensures that it will
never be used with stacking.

Refactor to slightly simplify the code in x_to_label(), this
also fixes a long standing problem where x_to_labels check on stacking
is only on the first element to the table option list, instead of
the element that is found and used.

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