Partially revert last Gnus merge

Partially revert last Gnus merge

The add-face function has been added gnus-compat.el, but is not in
Emacs; this leaves Gnus in an unusable state.

* lisp/gnus/gnus-salt.el (gnus-tree-highlight-node):
* lisp/gnus/gnus-sum.el (gnus-summary-highlight-line):
* lisp/gnus/gnus-group.el (gnus-group-highlight-line): Revert use of add-face.

* lisp/gnus/gnus-util.el
(gnus-put-text-property-excluding-characters-with-faces): Restore.