http.
* openfile.c (overwrite_filep, make_outfile_name, open_outfile)
(open_sigfile): Use iobuf_is_pipe_filename to check for pipes so
that special filesnames are taken into account. This is bug 327.
* tdbdump.c (import_ownertrust): Ditto.
* sign.c (write_plaintext_packet): Ditto.
* progress.c (handle_progress): Ditto.
* plaintext.c (handle_plaintext): Ditto.
* encode.c (encode_simple, encode_crypt): Ditto.
* iobuf.c (iobuf_is_pipe_filename): New.
* fileutil.c (is_file_compressed): Use it here.