numfmt: add --unit-separator

numfmt: add --unit-separator

Output, accept, or disallow a string between the number and unit
as recommended in <https://physics.nist.gov/cuu/Units/checklist.html>
I.e. support outputting numbers of the form: "1234 M"

* src/numfmt.c (simple_strtod_human): Skip unit separator if present,
or disallow a unit separator if empty.
(double_to_human): Output unit separator if specified.
(main): Accept --unit-separator.
* tests/misc/numfmt.pl: Add test cases.
* doc/coreutils.texi: Describe the new option,
giving examples of interaction with --delimiter.
* NEWS: Mention the new feature.
* THANKS.in: Add Johannes Schauer Marin Rodrigues,
who provided a preliminary patch.