Fix POSIX-conformance bug: "sort -k 3,3.5b" is supposed to skip

Fix POSIX-conformance bug: "sort -k 3,3.5b" is supposed to skip
leading blanks when computing the location of the field end;
it is not supposed to skip trailing blanks.  Solaris 8 "sort"
does conform to POSIX.  Also fix the documentation to clarify
this and related issues.

(limfield): Use skipeblanks, not skipsblanks, to
decode whether to skip leading blanks.
(trailing_blanks): Remove.
(fillbuf, getmonth, keycompare): Don't trim trailing blanks.