Adapt shadowfile.el for Tramp (Bug#4526, Bug#4846)

Adapt shadowfile.el for Tramp  (Bug#4526, Bug#4846)

* etc/NEWS: Mention changes in shadowfile.el.

* lisp/shadowfile.el (top): Require 'tramp instead of 'ange-ftp.
(shadow-cluster): New defstruct.
(shadow-make-cluster, shadow-cluster-name, shadow-cluster-primary)
(shadow-cluster-regexp, shadow-get-user)
(shadow-parse-fullname): Remove.
(shadow-info-file, shadow-todo-file, shadow-system-name)
(shadow-homedir, shadow-regexp-superquote, shadow-suffix)
(shadow-set-cluster, shadow-get-cluster, shadow-site-name)
(shadow-name-site, shadow-site-primary, shadow-site-cluster)
(shadow-read-site, shadow-parse-name, shadow-make-fullname)
(shadow-replace-name-component, shadow-local-file)
(shadow-expand-cluster-in-file-name, shadow-contract-file-name)
(shadow-same-site, shadow-file-match, shadow-define-cluster)
(shadow-define-literal-group, shadow-define-regexp-group)
(shadow-make-group, shadow-shadows-of-1, shadow-read-files)
(shadow-write-info-file, shadow-write-todo-file)
(shadow-initialize): Adapt variables and functions.

* test/lisp/shadowfile-tests.el: New file.