get rid of undefined behavior in union member mismatch for $@
The code for $@ depends on reading a_w.w_symbol values, but the other A_DOLLAR branch (i.e., for $1 - $n) depend on reading a_w.w_index. So for the conditional at the top of binbuf_eval, this means that any normal A_DOLLAR that has its a_w.w_index set is actually trying to read a_w.w_symbol. On a 64-bit arch with certain compilers, this will result in a read of uninitalized data in the padding of the part of w_symbol that exceeds the sizeof(int). The workaround here is to define a sentinel value DOLLARALL to a negative int unlikely to ever be used in real A_DOLLAR values. We then use DOLLARALL to mean the "@" in "$@" so we can create a special branch for it in the parser. One consequence-- we probably need to add a special case error when this value is out of range. So rather than confusing the user with the mysterious negative value of DOLLARALL we print "$@" for them. (Currently, the only place I can see this happening is when "$@" is used...