forbid the use of std::endl on std::cerr

std::cerr will flush after each operator<< by default, so it's simpler
to use \n instead of std::endl, especially if we can merge \n into the
previous string.  Ideally we should prefer \n for std::cout as well,
but there are reasonable cases where we want to call std::endl there,
so it's hard to enforce.

* tests/sanity/style.test: Diagnose occurrences of cerr.*<<.*endl.
* bin/autcross.cc, bin/autfilt.cc, bin/ltlcross.cc, bin/ltlsynt.cc,
spot/tl/formula.cc, spot/twa/bdddict.cc, tests/core/checkpsl.cc,
tests/core/checkta.cc, tests/core/consterm.cc, tests/core/emptchk.cc,
tests/core/equalsf.cc, tests/core/ikwiad.cc, tests/core/kind.cc,
tests/core/length.cc, tests/core/ltlrel.cc, tests/core/parity.cc,
tests/core/randtgba.cc, tests/core/reduc.cc, tests/core/syntimpl.cc,
tests/ltsmin/modelcheck.cc: Fix them.

bin/autfilt.cc

@@ -769,7 +769,7 @@ parse_opt(int key, char* arg, struct argp_state*)
                     {
                       std::cerr << ", '" << acc_is_args[i] << '\'';
                     }
-                std::cerr << std::endl;
+                std::cerr << '\n';
                 exit(2);
               }
           }