sort: avoid redundant processing with inaccessible inputs or output

* src/sort.c (check_inputs): A new function to verify all inputs
are accessible before further processing.
(check_output): A new function to open or create a specified
output file, before futher processing.
(stream_open): Adjust to truncating the previously opened
output file rather than opening directly.
(avoid_trashing_input): Optimize to stat the output file
descriptor, rather than the file name.
(main): Call the new functions to check accessibility of
inputs and output, before processing starts.
* tests/misc/sort: Adjust to the changed error message.
* tests/misc/sort-merge-fdlimit: Account for the earlier opened
file descriptor of the specified output file.
* tests/misc/sort-exit-early: A new test to exercise the improvements.
* tests/Makefile.am: Reference the new test.
* NEWS: Mention the improvement.

Suggested-by: Bernhard Voelker

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