(find-auto-coding): Provide filename to `auto-coding-functions`

(find-auto-coding): Provide filename to `auto-coding-functions`

Allow `auto-coding-functions` to know the file name.
Motivated by the needs of Editorconfig support.

* lisp/international/mule.el (auto-coding-file-name): New var.
(find-auto-coding): Let-bind it for `auto-coding-functions`.
Document the expectation that the arg be an absolute file name.

* doc/lispref/nonascii.texi (Default Coding Systems):
Mention `auto-coding-file-name`.

* test/lisp/international/mule-util-resources/test.utf-16le: New file.
* test/lisp/international/mule-tests.el (mule-tests--dir): New var.
(mule-tests--auto-coding): New fun.
(mule-tests--auto-coding-functions): New test.