numfmt: support reading numbers with NBSP before unit

numfmt: support reading numbers with NBSP before unit

* src/numfmt.c (simple_strtod_human): Accept (multi-byte)
non-breaking space character between number and unit.
Note we restrict this to a single character between number
and unit, to allow less ambiguous parsing if multiple blanks
are used to delimit fields.
* tests/misc/numfmt.pl: Add test cases.
* doc/coreutils.texi (numfmt invocation): Fix stale description
--delimiter skipping whitespace.
* NEWS: Mention the improvement.