sort: --debug: output data independent warnings and info

* src/sort.c (usage): Mention --debug can output warnings to stderr.
Also split the translatable string to aid translation.
(default_key_compare): A new function refactored from main(),
and now also called from the new key_warnings() function.
(key_to_opts): A new function refactored from incompatible_options(),
and now also called from the new key_warnings() function.
(key_numeric): A new function refactored to test if key is numeric.
(key_warnings): A new function to output warnings to stderr,
about questionable use of various options.  Currently it warns
about zero length keys and ineffective global options.
(incompatible_options): Refactor out key_to_opts()
(main): Use key_init() to initialize gkey.  Refactor out
default_key_compare().  Call key_warnings() in debug mode.
* doc/coreutils.texi (sort invocation): Mention that warnings
are output by --debug.
* tests/misc/sort-debug-warn: A new test for debug warnings.
* tests/Makefile.am: Reference the new test.
* NEWS: Mention the new feature

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