Implement SELINUX backends.

Implement SELINUX backends.

* net/tramp.el (tramp-file-name-handler-alist): Add
`file-selinux-context' and `set-file-selinux-context'.
(tramp-handle-file-selinux-context)
(tramp-handle-set-file-selinux-context): New defuns.
(tramp-handle-copy-file, tramp-do-copy-or-rename-file): Handle
PRESERVE-SELINUX-CONTEXT.

* net/tramp-gvfs.el (tramp-gvfs-file-name-handler-alist): Add
`file-selinux-context' and `set-file-selinux-context'.
(tramp-gvfs-handle-file-selinux-context)
(tramp-gvfs-handle-set-file-selinux-context): New defuns.
(tramp-gvfs-handle-copy-file): Handle PRESERVE-SELINUX-CONTEXT.

* net/ange-ftp.el (ange-ftp-copy-file):
* net/tramp-fish.el (tramp-fish-handle-copy-file):
* net/tramp-imap.el (tramp-imap-handle-copy-file):
* net/tramp-smb.el (tramp-smb-handle-copy-file): Add
PRESERVE-SELINUX-CONTEXT.