Fix state cleanup bug between rule/config parser runs.

This fixes a bug that has been annoying me minorly for some time now:
sometimes, after parse errors, a subsequent parser run would fail.  The reason
is that yylex() modifies some global variables (yytext, yydata) during its run
to keep state. To make subsequent parser runs correct, these have to be reset
before each run.

Also, close files after reading them.