Fix initialisation of lexer.

A forgotten fseek; took ages to figure out.
Tokeniser can be regarded correct.

For some small values of correct:  I have switched from using wchar to
char, for the sake of simplicity.  Until the parser is completed, I'll
stick to chars.  When the think is working, and there are enough tests,
I'll proceed to using wchars.

Unicode, in C, is hard, even in this kind of trivial scenario.