sort: use the more efficient posix_spawn to invoke --compress-program

sort: use the more efficient posix_spawn to invoke --compress-program

* NEWS: Mention the improvement. Mention that 'sort' will continue
without compressing temporary files if the program specified by
--compress-program cannot be executed.
* doc/coreutils.texi (sort invocation): Document the behavior when the
program specified by --compress-program cannot be executed.
* src/sort.c: Include spawn.h.
(MAX_FORK_TRIES_COMPRESS, MAX_FORK_TRIES_DECOMPRESS): Remove definition.
(MAX_TRIES_COMPRESS, MAX_TRIES_DECOMPRESS): New definitions based on
MAX_FORK_TRIES_COMPRESS and MAX_FORK_TRIES_DECOMPRESS.
(async_safe_die): Remove function.
(posix_spawn_file_actions_move_fd): New function.
(pipe_fork): Remove function.
(pipe_child): New function based on pipe_fork. Return an error number
instead of a pid. Use posix_spawnp instead of calling fork and expecting
the caller to exec.
(maybe_create_temp): Call pipe_child instead of pipe_fork. Print a
warning to standard error if --compress-program cannot be executed and
the error is different than the previous call. Remove code for the child
process.
(open_temp): Remove code for the child process. Improve error message.
* tests/sort/sort-compress.sh: Add a test case for when the program
specified by --compress-program does not exist.