tests: change all-fields specifier

* tests/misc/numfmt.pl: Per Pádraig's suggestion, have a lone '-'
passed to --fields be the all-fields specifier. '*' conflicts with
shell globbing, and a separate --all-fields option is redundant. '-'
fits nicely with the existing N-M range specifier pattern and can be
taken to mean an 'unbounded' range, since there is no start or end.
numfmt: use '-' for all-fields specifier

* src/numft.c: (parse_field_arg): if optarg is a single dash set
the ALL_FIELDS processing flag to true.
(usage): replace '*' with '-' as all fields specifier
Option --all-fields removed, just use field specifier