cex: don't assume the terminal supports "•"

Use of print_unicode_char suggested by Bruno Haible.
https://lists.gnu.org/r/bug-gettext/2020-06/msg00012.html

* src/gram.h (print_dot_fallback, print_dot): New.
* src/gram.c, src/derivation.c: Use it.
* tests/counterexample.at, tests/report.at: Adjust the test suite.
* .travis.yml, README-hacking.md: Adjust.

TODO

@@ -35,6 +35,12 @@ Unless we play it dumb (little structure).
 *** Doc
 -Wcounterexamples, --report=counterexamples
 
+Use "•" instead of ".".
+
+*** Conflict coverage
+Not all the conflicts have counterexamples generated.  See the "break"s in
+counterexample_report_state.
+
 ** glr.cc
 Get rid of global_tokens_and_yystype.