Fixes issue #553.

* spot/twaalgos/strength.cc (is_type_automaton): Make sure an
accepting SCC is not followed by a rejecting one.
(is_terminal_automaton): Mark the third-argument version deprecated.
* spot/twaalgos/strength.hh: Adjust.
* spot/twaalgos/couvreurnew.cc: Remove the inappropriate terminal
optimization.
* bin/ltlfilt.cc, spot/tl/hierarchy.cc, spot/twaalgos/gfguarantee.cc,
tests/core/ikwiad.cc: Remove usage of the third argument of
is_terminal_automaton.
* tests/core/readsave.test, tests/core/strength.test: Adjust test
cases.
* NEWS: Mention the bug.