* common/sysutils.h (gnupg_check_special_filename): New.
* common/sysutils.c (gnupg_check_special_filename): New.
* common/iobuf.c (translate_file_handle): Remove.
(iobuf_is_pipe_filename): Use gnupg_check_special_filename.
(do_open): Use gnupg_check_special_filename.
* g10/plaintext.c (get_output_file): Use gnupg_check_special_filename
and open_stream_nc.