Add strings to bison grammar tokens.

This ensures that any syntax errors passed to the parser are printed
using the string reprentations of the tokens rather than their name.

New:
    SDC Error line 16 near '-max': syntax error, unexpected -max, expecting -from or -to or EOL

Old:
    SDC Error line 16 near '-max': syntax error, unexpected ARG_MAX, expecting ARG_FROM or ARG_TO or EOL