cut: refactor into set-fields module

cut: refactor into set-fields module

Extract the functionality of parsing --field=LIST into a separate
module, to be used by other programs.

* src/cut.c: move field parsing code from here ...
* src/set-fields.{c,h}: ... to here.
  (set_fields): generalize by supporting multiple parsing/reporting
  options.
  (struct range_pair): rename to field_range_pair.
* src/local.mk: link cut with set-field.
* po/POTFILES.in: add set-field.c
* tests/misc/cut.pl: update wording of error messages