(_GNU_SOURCE): Remove; autoconf now does this.

(_GNU_SOURCE): Remove; autoconf now does this.
(child_open_for_compress, child_open_for_uncompress, close_archive):
Propagate any failure of the compression process back to "tar".
(open_archive, flush_write, flush_read, close_archive):
Do not allocate an array of size PATH_MAX, as PATH_MAX might be (size_t) -1.
Instead, allocate an array with the size that's needed.
(open_archive): Don't bother checking S_ISCHR of /dev/null.
(backspace_output): Don't try to backspace past start of archive.
(close_archive): Remove special case for DELETE_SUBCOMMAND.