Make REs in magic-(fallback-)mode-alist case-sensitive.

Make REs in magic-(fallback-)mode-alist case-sensitive.

These variables are used for well-defined file formats where relaxed
case matching is not wanted usually.

* lisp/files.el (magic-mode-alist, magic-fallback-mode-alist): Update
the doc string.
(set-auto-mode): Make looking-at for elements of magic-mode-alist and
magic-fallback-mode-alist use case-fold-search == nil.
* lisp/files.el (files-test-magic-mode-alist-re-baseline)
(files-test-magic-mode-alist-re-no-match)
(files-test-magic-mode-alist-re-case-diff): Add.