xgettext: Define alternate parser signature for the Ruby support.
* gettext-tools/src/xgettext.c (extract_from_stream_func): Renamed from
extractor_func.
(extract_from_file_func): New type.
(struct extractor_ty): Rename field func to extract_from_stream. New field
extract_from_file.
(main): Consider both fields.
(xgettext_find_file): New function.
(extract_from_file): Add alternative code for when the other extraction function
is defined.
(language_to_extractor): In the table, rename field func to extract_from_stream,
and add new field extract_from_file.
* gettext-tools/src/x-*.h (SCANNERS_*): Update.
