sort: fix parsing of end field in obsolescent key formats

sort: fix parsing of end field in obsolescent key formats

This regression was introduced in commit 224a69b5, 2009-02-24,
"sort: Fix two bugs with determining the end of field".
The specific regression being that we include 1 field too many when
an end field is specified using obsolescent key syntax (+POS -POS).

* src/sort.c (struct keyfield): Clarify the description of the eword
member, as suggested by Alan Curry.
(main): When processing obsolescent format key specifications,
normalize eword to a zero based count when no specific end char is given
for an end field. This matches what's done when keys are specified with -k.
* tests/misc/sort: Add a few more tests for the obsolescent key formats,
with test 07i being the particular failure addressed by this change.
* THANKS: Add Alan Curry who precisely identified the issue.
* NEWS: Mention the fix.
Reported by Santiago Rodríguez