sort: file descriptor discipline

sort: file descriptor discipline

Use O_CLOEXEC when creating file descriptors, so that subsidiary
processes do not inherit file descriptors that they do not need.
This is helpful for ‘sort’, as it is a multithreaded program that
forks and execs.
* bootstrap.conf (gnulib_modules): Add mkostemp, open, pipe2.
* src/sort.c (create_temp_file): Open temporary file with O_CLOEXEC.
(stream_open): Open the stream with O_CLOEXEC.
(pipe_fork): Create the pipe with O_CLOEXEC.
(check_output): Open the output file with O_CLOEXEC.
(main): Use xfopen/xfclose to handle --files0-from, so that
O_CLOEXEC is used properly.  This is simpler anyway.
* tests/misc/sort-files0-from.pl: Adjust to change in diagnostic
wording.