(write_archive_buffer): New function.

(write_archive_buffer): New function.
(child_open_for_compress, flush_write, flush_read): Use it to write
buffers.
(open_archive): Report error if fstat of archive fails.
Improve efficiency of check for /dev/null.
Also, fix some corner cases with remote archives and /dev/null checking.
(close_archive): Test for input fifo only if not remote.
Truncate output archive only if it's not remote.