Fixes #212.

* spot/parseaut/parseaut.yy, spot/twaalgos/hoa.cc: Recognize
exist-branch, and adjust printer to the 1.1 semantics.
* tests/core/alternating.test, tests/core/complete.test,
tests/core/det.test, tests/core/explsum.test,
tests/core/parseaut.test, tests/core/readsave.test,
tests/core/sbacc.test, tests/core/tgbagraph.test,
tests/python/alternating.py, tests/python/dualize.py: Adjust test
cases.
* NEWS: Mention the change.