config_file.l: Instruct flex to create a reentrant scanner.

This requires some modifications:

* Most functions take an additional yyscan_t argument.
* We must call yylex_init and yylex_destroy().
* Accessor methods must be used to get yytext and yylen.

We used to set the initial start condition in lls_convert_config(),
before the call to yylex(). For reentrant scanners, however,
the BEGIN macro can not be used in the user code section where
lls_convert_config() is located. But pushing a state works and should
be equivalent.

The generated scanner is not reentrant yet because it still has global
variables. This issue will be addressed in a subsequent patch.