Fix tramp-sshfs

Fix tramp-sshfs

* lisp/net/tramp-fuse.el (tramp-fuse-handle-file-exists-p): New defun.
(tramp-fuse-mount-timeout): Move up.
(tramp-fuse-mount-point): Use `tramp-fuse-mount-timeout'.
(tramp-fuse-unmount): Flush "mount-point" file property.

* lisp/net/tramp-sshfs.el (tramp-sshfs-file-name-handler-alist):
Use `tramp-fuse-handle-file-exists-p'.

* test/lisp/net/tramp-tests.el (all): Use function read syntax
where appropriate.
(tramp-test39-detect-external-change): Let-bind
`read-from-minibuffer' instead of `yes-or-no-p'.