Add file encryption to Tramp

Add file encryption to Tramp

* lisp/net/tramp-crypt.el: New file.

* lisp/net/tramp.el (tramp-run-real-handler):
Add `tramp-crypt-file-name-handler'.
(tramp-register-file-name-handlers):
Call `tramp-register-crypt-file-name-handler'.
(tramp-handle-insert-file-contents, tramp-local-host-p): Check for
`tramp-crypt-enabled'

* test/lisp/net/tramp-tests.el (tramp--test-crypt-p): New defun.
(tramp-test24-file-acl, tramp-test25-file-selinux)
(tramp-test28-process-file, tramp-test29-start-file-process)
(tramp-test30-make-process, tramp-test31-interrupt-process)
(tramp-test32-shell-command)
(tramp-test32-shell-command-dont-erase-buffer)
(tramp-test33-environment-variables)
(tramp-test33-environment-variables-and-port-numbers)
(tramp-test34-explicit-shell-file-name, tramp-test35-exec-path)
(tramp-test35-remote-path, tramp-test36-vc-registered)
(tramp--test-check-files, tramp-test43-asynchronous-requests): Use it.