Make sure we only output one initial state in LBTT's output.

* src/tgbaalgos/lbtt.cc (fill_todo): Add the 'first' argument
to designate initial states.
(lbtt_reachable): Adjust calls to fill_todo.  Handle the
fake initial state accepting conditions specially.
* src/tgbaalgos/lbtt.hh: Update comments.

src/tgbaalgos/lbtt.hh

@@ -12,16 +12,19 @@ namespace spot
   /// labeled by propositional formulae, and generalized
   /// Büchi accepting conditions on \emph states.  This
   /// is unlike our spot::tgba automata which put
-  /// botg generalized accepting conditions and propositional
+  /// both generalized accepting conditions and propositional
   /// formulae) on \emph transitions.
   ///
-  /// This algorithm will therefore produce an automata
-  /// where accepting conditions have been moved from
-  /// each transition to previous state.  In the worst
-  /// case, doing so will multiply the number of states
-  /// and transitions of the automata by <code>2^|Acc|</code>.
-  /// where <code>|Acc|</code> is the number of accepting
-  /// conditions used by the automata.  You have been warned.
+  /// This algorithm will therefore produce an automata where
+  /// accepting conditions have been moved from each transition to
+  /// previous state.  In the worst case, doing so will multiply the
+  /// number of states and transitions of the automata by
+  /// <code>2^|Acc|</code>.  where <code>|Acc|</code> is the number of
+  /// accepting conditions used by the automata.  (It can be a bit
+  /// more because LBTT allows only for one initial state:
+  /// lbtt_reachable() may also have to create an additional state in
+  /// case the source initial state had to be split.)  You have been
+  /// warned.
   ///
   /// \param g The automata to print.
   /// \param os Where to print.