grammar: record used associativity and print useless ones

Record which symbol associativity is used, and display useless ones.

* src/symtab.h, src/symtab.c (register_assoc, print_assoc_warnings): New
* src/symtab.c (init_assoc, is_assoc_used): New
* src/main.c: Use print_assoc_warnings
* src/conflicts.c: Use register_assoc
* tests/conflicts.at (Useless associativity warning): New.

Due to the new warning, many tests had to be updated.

* tests/conflicts.at tests/existing.at tests/regression.at:
Add the associativity warning in the expected results.
* tests/java.at: Fix the java calculator's grammar to remove a useless
associativity.
* doc/bison.texi (mfcalc example): Fix associativity to remove
warning.

tests/java.at

@@ -54,11 +54,11 @@ AT_DATA([Calc.y],
 %token <Integer> NUM "number"
 %type  <Integer> exp
 
-%nonassoc '=' /* comparison            */
+%nonassoc '='       /* comparison            */
 %left '-' '+'
 %left '*' '/'
-%left NEG     /* negation--unary minus */
-%right '^'    /* exponentiation        */
+%precedence NEG     /* negation--unary minus */
+%right '^'          /* exponentiation        */
 
 /* Grammar follows */
 %%