* tests/gpgscm/scheme-private.h (struct port): Use boxed values for
filename and current line.  This allows us to use the same Scheme
object for labeling all expressions in a file.
* tests/gpgscm/scheme.c (file_push): Use boxed type for filename.
(mark): Mark location objects of port objects.
(gc): Mark location objects in the load stack.
(port_clear_location): New function.
(port_reset_current_line): Likewise.
(port_increment_current_line): Likewise.
(file_pop): Adapt accordingly.
(port_rep_from_filename): Likewise.
(port_rep_from_file): Likewise.
(port_close): Likewise.
(skipspace): Likewise.
(token): Likewise.
(_Error_1): Likewise.
(opexe_0): Likewise.
(opexe_5): Likewise.
(scheme_deinit): Likewise.
(scheme_load_file): Likewise.
(scheme_load_named_file): Likewise.