join: make -t '' operate on the whole line

join: make -t '' operate on the whole line

Previously passing an empty parameter to -t would
raise an error, but now it means to treat each line
as a single field for matching.  This matches the
default operation of `sort` which is usually used
in conjunction with join.

* src/join.c (main): Set the field delimiter to '\n' if
an empty parameter is passed to -t.
(usage): Mention the operation of -t ''.
* tests/misc/join: Add 2 new tests, for the existing -t '\0'
and the new -t '' functionality.
* doc/coreutils.texi (join invocation): Mention that
join -t '' always operates on the whole line, while
join -t '\0' usually does.
* NEWS: Mention the change in behavior.