string ports simplification

string ports simplification

* libguile/strports.c (st_fill_input): Rename from stfill_buffer, and
  remove an unneeded scm_return_first_int.
  (st_resize_port): Minor variable renaming.
  (st_write): Keep read_pos updated to be the same as write_pos.  If we
  need to resize, do so only once.
  (st_seek): No more need to flush.
  (st_truncate): Update read_pos here too.
  (scm_mkstrport): No need to flush here.
  (scm_strport_to_string): Just call scm_from_stringn; rely on it to
  detect the latin1 case.
  (scm_make_stptob): No more flush function.