Add 7z archive format support (bug#5475).

Add 7z archive format support (bug#5475).

* arc-mode.el (archive-zip-extract): Try to find 7z executable.
(archive-7z-extract): New defcustom.
(archive-find-type): Add magic string for 7z.
(archive-extract-by-stdout): Add new optional arg `stderr-file'.
If `stderr-file' is non-nil, use `(t stderr-file)' for the
`buffer' arg of `call-process'.
(archive-zip-extract): Check `archive-zip-extract' for "7z" and
call the function `archive-7z-extract' with the variable
`archive-7z-extract' let-bound to `archive-zip-extract'.
(archive-7z-summarize, archive-7z-extract): New functions.

* international/mule.el (auto-coding-alist):
* files.el (auto-mode-alist): Add 7z file extension.