sort: simplify -o handling to avoid fdopen, assert

sort: simplify -o handling to avoid fdopen, assert

* src/sort.c (outfd): Remove.  All uses replaced by STDOUT_FILENO.
(stream_open): When writing, use stdout rather than fdopen.
(move_fd_or_die): Renamed from dup2_or_die, with the added functionality
of closing its first argument.  All uses changed.
(avoid_trashing_input): Special case for !outfile no longer needed.
(check_output): Arrange for standard output to go to the file,
rather than storing the fd in outfd.