tests: partial output can occur before an error

* tests/misc/numfmt.pl: Each field is now processed and printed
in turn, so there may be some output by the time a field is reached that
causes an error.  Update the tests to reflect this.  Also remove test
for the 'invalid' field -5.. this is now a valid range specifier.
doc: add usage for field range specifiers

* src/numfmt.c (usage): Document newly supported field range specifiers
  and --all-fields option