stats: fix slow %s and inappropriate %S output

Fixes #269.

* spot/twaalgos/stats.cc: Use twa_statistics instead of
twa_sub_statistics when %t is not used.
* bin/common_aoutput.cc: Likewise, also fix %S to use twa_statistics
instead of num_states(), and document that %s,%t,%e all return
statistics about the reachable part of the automaton.
* tests/core/format.test: Add more tests.
* NEWS: Document the issue.

NEWS

@@ -11,6 +11,12 @@ New in spot 2.3.4.dev (not yet released)
     SCC) or 'y' (split universal destination by colors) universal
     edges could be connected to undefined states.
 
+  - Using --stats=%s or --stats=%s or --stats=%t could take an
+    unnecessary long time on automata with many atomic propositions,
+    due to a typo.  Furthermore, %s/%e/%t/%E/%T were printing
+    a number of reachable states/edges/transitions, but %S was
+    incorrectly counting all states even unreachable.
+
 New in spot 2.3.4 (2017-05-11)
 
   Bugs fixed: