Amend hack-local-variables-prop-line not always to return any mode on line 1.

Amend hack-local-variables-prop-line not always to return any mode on line 1.

This fixes bug #24266.

* lisp/files.el (hack-local-variables-prop-line): Change the name of the
parameter mode-only to handle-mode.  Change its meaning, such that it being
set to a value non-nil and not t removes any mode parameter from the result
list.  Leave its values nil and t with the same meanings they had.
(hack-local-variables): Call hack-local-variables-prop-line appropriately.