... instead of inherently-weak.  The reason they were tagged
as inherently-weak is historical: this property was introduced
1.5 years before the weak propery.

Fixes #351.

* spot/twaalgos/ltl2tgba_fm.cc: Use prop_weak() instead of
prop_inherently_weak().  Also be more conservative about the use of
single_acc when unambiguous automata are generated.