config_file.l: Move helper functions to user code section.

config_file.l: Move helper functions to user code section.

These functions refer to the global variables yytext and yylen.
Reentrant scanners need to use accessor methods instead of accessing
the global variables directly. But these methods are not available
in the definitions section.

As a preparation for making the scanner reentrant, this patch moves
the three helper functions from the definitions section to the user
code section. Since the functions are called from the actions of the
pattern rules, we need to provide forward declarations.

Pure code movement, no actual changes.