variable-parser: Rewrite parse_DATA_LIST_vars().

variable-parser: Rewrite parse_DATA_LIST_vars().

This rewrite was prompted by getting rid of the VAR_NAME_LEN limit inside
parse_DATA_LIST_vars(), but then I noticed that the variable naming and
coding style was dated, and that duplicate variable names were only
detected for variables named using TO, not for individual names, so I
rewrote much of the code instead.