The buffer length len and the requested number of tokens n are mixed up
in an invalid comparison. This causes all valid requests for n < len
tokens to be denied and all invalid requests for n > len tokens to be
accepted. This may cause a buffer overflow if the caller requests more
characters than they provide space for.
The linked list is doubly linked so the parser can look forward into it
and error reporting can look backward.
This commmit also reworks main to use the tokenlist instead of dealing
with the lexer manually.
