Add missing handler to tramp-rclone.el, improve robustness

Add missing handler to tramp-rclone.el, improve robustness

* lisp/net/tramp-rclone.el (tramp-adb): Require.
(tramp-rclone-file-name-handler-alist):
Use `tramp-adb-handle-expand-file-name'.
(tramp-rclone-flush-directory-cache): New defun, derived from
`tramp-rclone-flush-mount'.
(tramp-rclone-do-copy-or-rename-file)
(tramp-rclone-handle-delete-directory)
(tramp-rclone-handle-delete-file)
(tramp-rclone-handle-make-directory): Use it.
(tramp-rclone-handle-directory-files)
(tramp-rclone-local-file-name):
Use `tramp-compat-file-name-quoted-p', 	`tramp-compat-file-name-quote'
and ´tramp-compat-file-name-unquote'.
(tramp-rclone-handle-file-executable-p)
(tramp-rclone-handle-file-readable-p): Cache result.
(tramp-rclone-handle-file-name-all-completions)
(tramp-rclone-mounted-p, tramp-rclone-remote-file-name)
(tramp-rclone-maybe-open-connection): Rewrite.

* test/lisp/net/tramp-tests.el (tramp--test-rclone-p): New defun.
(tramp-test05-expand-file-name-relative)
(tramp--test-special-characters): Use it.