Warn about unset midrule $$ if the corresponding $n is used.

Warn about unset midrule $$ if the corresponding $n is used.
* src/reader.c (symbol_should_be_used): Check midrule parent rule for
$n usage.
(packgram): Before invoking grammar_rule_check on any rule, make sure
all actions have already been scanned in order to set `used' flags.
Otherwise, checking that a midrule's $$ is set will not always work
properly because the midrule check must forward-reference the midrule's
parent rule.
* tests/input.at (AT_CHECK_UNUSED_VALUES): Extend to check the new
warning.