Handle case-insensitivity for safe-local-variable-directories.

Handle case-insensitivity for safe-local-variable-directories.

* lisp/emacs-lisp/files.el (hack-local-variables-filter): Use
'file-equal-p' when checking 'safe-local-variable-directories'.
* doc/lispref/variables.texi (File Local Variables): Remove sentences
in 'safe-local-variable-directories' description about case-sensitivity
and trailing slash behaviors.
* doc/emacs/custom.texi (Safe File Variables): Remove sentence about
'safe-local-variable-directories' trailing slash behavior.
(Bug#61901)