* src/reader.c (grammar_current_rule_check): Also check that $$

* src/reader.c (grammar_current_rule_check): Also check that $$
is used.
Take the rule to check as argument, hence rename as...
(grammar_rule_check): this.
* src/reader.h, src/reader.c (grammar_rule_begin, grammar_rule_end):
Rename as...
(grammar_rule_begin, grammar_rule_end): these, for consistency.
(grammar_midrule_action, grammar_symbol_append): Now static.
* tests/torture.at (input): Don't rely on the default action
being always performed.
* tests/calc.at: "Set" $$ even when the action is "cut" with
YYERROR or other.
* tests/actions.at (Exotic Dollars): Instead of using unused
values, check that the warning is issued.