Improve SELinux handling in Tramp

Improve SELinux handling in Tramp

* lisp/net/tramp-sh.el (tramp-stat-file-attributes-with-selinux)
(tramp-stat-directory-files-and-attributes-with-selinux): New defconst.
(tramp-do-file-attributes-with-ls)
(tramp-do-file-attributes-with-stat)
(tramp-do-directory-files-and-attributes-with-stat): Return also
SELinux context.
(tramp-remote-selinux-p, tramp-do-copy-or-rename-file): Adapt docstring.

* lisp/net/tramp-sudoedit.el (tramp-sudoedit-do-copy-or-rename-file)
(tramp-sudoedit-remote-selinux-p): Adapt docstring.
(tramp-sudoedit-file-attributes-with-selinux): New defconst.
(tramp-sudoedit-handle-file-attributes): Use it.

* lisp/net/tramp.el (tramp-convert-file-attributes):
Extract SELinux context.