common: In private key mode write "Key:" always last in name-value.

* common/name-value.c (nvc_write): Take care of Key. Factor some code
out to ...
(write_one_entry): new.

The key item is in general not manual editable thus we put it at the
end of a file.

(cherry picked from commit c9fa28bfad297b17e76341ffb40383ce92da5d44)

• Signed-off-by: Werner Koch <wk@gnupg.org>