sort: fix silent exit upon SIGPIPE from --compress-program

sort: fix silent exit upon SIGPIPE from --compress-program

* src/sort.c (main): Ignore SIGPIPE so we've more control over
how we handle for stdout and compression programs.
(sort_die): Handle EPIPE from stdout and mimic a standard SIGPIPE,
otherwise reverting to a standard exit(SORT_FAILURE);
* tests/sort/sort-compress-proc.sh: Add a test case.
* NEWS: Mention the bug fix.