Let the handling of the skeleton files be local to the procedures

Let the handling of the skeleton files be local to the procedures
that use it.
* src/files.c (xfopen, xfclose, skeleton_find, guardfile): No
longer static.
(fparser, open_extra_files): Remove.
(open_files, output_files): Don't take care of fparser.
* src/files.h: Adjust.
* src/output.c (output_parser): Open and close the file to the
skeleton.
* src/reader.c (read_declarations): When %semantic_parser, open
fguard.