sort: avoid issues when issuing diagnostics from child processes

sort: avoid issues when issuing diagnostics from child processes

* src/sort.c: (async_safe_die): A new limited version of error(),
that outputs fixed strings and unconverted errnos to stderr.
This is safe to call in the limited context of a signal handler,
or in this particular case, between the fork() and exec() of
a multithreaded process.
(move_fd_or_die): Use the async_safe_die() rather than error().
(maybe_create_temp): Likewise.
(open_temp): Likewise.
Fixes http://bugs.gnu.org/15970