Add accessors for `file-attributes'

Add accessors for `file-attributes'

* doc/lispref/files.texi (File Attributes): Mention the accessors.

* lisp/files.el (file-attribute-type)
(file-attribute-link-number, file-attribute-user-id)
(file-attribute-group-id, file-attribute-access-time)
(file-attribute-modification-time)
(file-attribute-change-time, file-attribute-size)
(file-attribute-modes, file-attribute-inode-number)
(file-attribute-device-number): New functions.

* src/dired.c (Ffile_attributes): Mention the accessors (bug#22890).