Don't apply the default %destructor/%printer to an unreferenced midrule

Don't apply the default %destructor/%printer to an unreferenced midrule
value.  Mentioned at
<http://lists.gnu.org/archive/html/bison-patches/2006-09/msg00104.html>.
* src/symtab.c (dummy_symbol_get): Name all dummy symbols initially
like $@n instead of just @n so that the default %destructor/%printer
logic doesn't see them as user-defined symbols.
(symbol_is_dummy): Check for both forms of the name.
* src/reader.c (packgram): Remove the `$' from each midrule symbol
name for which the midrule value is referenced in any action.
* tests/actions.at (Default %printer and %destructor for mid-rule
values): New test.
* tests/regression.at (Rule Line Numbers, Web2c Report): Update output
for change to dummy symbol names.