join: add --header option to always output the first line

join: add --header option to always output the first line

This essentially allows one to use --check-order with headings.
Note join without --check-order will already handle the common case
where headings do match in each file, however using --check-order will fail
often when the header sorts after the first line of data.

Note also that this will join header lines from each file even if
they don't match, with headings from the first file being used.

* NEWS: Mention the new option.
* doc/coreutils.texi (join invocation): Describe the new option.
* src/join.c (usage): Likewise.
(join): Join the header lines unconditionally.
* tests/misc/join: Add 5 new tests.