sort: fix failure if sort's parent has ignored SIGCHLD

sort: fix failure if sort's parent has ignored SIGCHLD

* src/sort.c (main): Reset the SIGCHLD handler to the default
as otherwise wait() could return an error.
* tests/misc/sort-compress: Set the CHLD handler in a subshell
to SIG_IGN to ensure the sort command resets it to SIG_DFL.
* NEWS: Mention the fix.