dired-revert: save line numbers instead of positions

dired-revert: save line numbers instead of positions

Positions might change if the length of one dired header line
changes; this happen, for instance, if we add new files.
Instead, line numbers are invariant under shrinks/enlargements
of the file header.
https://lists.gnu.org/archive/html/emacs-devel/2017-07/msg01092.html
* lisp/dired.el (dired-save-positions): Save the line numbers at point.
(dired-restore-positions): Use forward-line to restore the original
position (Bug#27968).
* test/lisp/dired-tests.el (dired-test-bug27968): Add test.