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.

bootstrap.conf

@@ -46,7 +46,7 @@ gnulib_modules='
   rename
   spawn-pipe stdbool stpcpy strdup-posix strerror strverscmp
   timevar
-  unistd unistd-safer unlink unlocked-io
+  unicodeio unistd unistd-safer unlink unlocked-io
   update-copyright unsetenv verify
   warnings
   winsz-ioctl