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.

tests/conflicts.at

@@ -864,10 +864,10 @@ State 5
     Shift/reduce conflict on token OP:
         1 exp: exp OP exp .
         1 exp: exp . OP exp
-      Example                  exp OP exp • OP exp
-      First derivation         exp ::=[ exp ::=[ exp OP exp • ] OP exp ]
-      Example                  exp OP exp • OP exp
-      Second derivation        exp ::=[ exp OP exp ::=[ exp • OP exp ] ]
+      Example                  exp OP exp . OP exp
+      First derivation         exp ::=[ exp ::=[ exp OP exp . ] OP exp ]
+      Example                  exp OP exp . OP exp
+      Second derivation        exp ::=[ exp OP exp ::=[ exp . OP exp ] ]
 
 ]])
 
@@ -1207,10 +1207,10 @@ State 1
     Reduce/reduce conflict on token $end:
         3 num: '0' .
         4 id: '0' .
-      Example                  '0' •
-      First derivation         exp ::=[ num ::=[ '0' • ] ]
-      Example                  '0' •
-      Second derivation        exp ::=[ id ::=[ '0' • ] ]
+      Example                  '0' .
+      First derivation         exp ::=[ num ::=[ '0' . ] ]
+      Example                  '0' .
+      Second derivation        exp ::=[ id ::=[ '0' . ] ]
 
 
 
@@ -1755,10 +1755,10 @@ State 4
     Shift/reduce conflict on token 'a':
        10 reported_conflicts: . %empty
         8 reported_conflicts: . 'a'
-      First example            resolved_conflict • 'a'
-      First derivation         start ::=[ resolved_conflict reported_conflicts ::=[ • ] 'a' ]
-      Second example           resolved_conflict • 'a' 'a'
-      Second derivation        start ::=[ resolved_conflict reported_conflicts ::=[ • 'a' ] 'a' ]
+      First example            resolved_conflict . 'a'
+      First derivation         start ::=[ resolved_conflict reported_conflicts ::=[ . ] 'a' ]
+      Second example           resolved_conflict . 'a' 'a'
+      Second derivation        start ::=[ resolved_conflict reported_conflicts ::=[ . 'a' ] 'a' ]
 
 
 
@@ -1774,10 +1774,10 @@ State 5
     Reduce/reduce conflict on token 'a':
         8 reported_conflicts: 'a' .
         9 reported_conflicts: 'a' .
-      Example                  'a' •
-      First derivation         reported_conflicts ::=[ 'a' • ]
-      Example                  'a' •
-      Second derivation        reported_conflicts ::=[ 'a' • ]
+      Example                  'a' .
+      First derivation         reported_conflicts ::=[ 'a' . ]
+      Example                  'a' .
+      Second derivation        reported_conflicts ::=[ 'a' . ]
 
 
 
@@ -1959,10 +1959,10 @@ AT_CHECK([[cat input.output | sed -n '/^State 0$/,/^State 1$/p']], 0,
     Reduce/reduce conflict on token 'c':
        12 empty_c2: . %empty
        13 empty_c3: . %empty
-      Example                  • 'c'
-      First derivation         start ::=[ empty_c2 ::=[ • ] 'c' ]
-      Example                  • 'c'
-      Second derivation        start ::=[ empty_c3 ::=[ • ] 'c' ]
+      Example                  . 'c'
+      First derivation         start ::=[ empty_c2 ::=[ . ] 'c' ]
+      Example                  . 'c'
+      Second derivation        start ::=[ empty_c3 ::=[ . ] 'c' ]