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.

po/POTFILES.in

@@ -6,6 +6,7 @@ src/counterexample.c
 src/files.c
 src/fixits.c
 src/getargs.c
+src/gram.h
 src/gram.c
 src/graphviz.c
 src/i18n-strings.c