sort: new --human-numeric-sort option to sort KiB MB etc.

sort: new --human-numeric-sort option to sort KiB MB etc.

* NEWS: Document the new option
* doc/coreutils.texi (sort invocation): ditto
* src/sort.c (main): handle the new --human-numeric-sort option (-h).
(human_numcompare): A new function to compare SI and IEC suffixes
before falling back to the standard --numeric comparison.
(find_unit_order): A new helper function to find the order
of magnitude of a number string as determined by its suffix.
(check_mixed_SI_IEC): A new helper function to exit with error
if both SI and IEC suffixes are presented.
* tests/misc/sort: Add 8 tests to test the new functionality.
* THANKS: Update