Futher tramp-crypt implementation and documentation

Futher tramp-crypt implementation and documentation

* doc/misc/tramp.texi (Top, Configuration): Insert section
`Keeping files encrypted' in menu.
(Keeping files encrypted): New node.

* lisp/net/tramp-crypt.el (tramp-crypt-file-name-handler-alist):
Add `tramp-set-file-uid-gid'.
(tramp-crypt-maybe-open-connection): Simplify.
(tramp-crypt-do-encrypt-or-decrypt-file): Use `binary' coding system.
(tramp-crypt-handle-set-file-uid-gid): New defun.

* test/lisp/net/tramp-tests.el (tramp-test09-insert-file-contents):
Adapt test.