Tramp code cleanup

Tramp code cleanup

* lisp/net/tramp.el (tramp-remote-path): Adapt docstring.
(tramp-action-login, tramp-action-password, tramp-action-yesno)
(tramp-action-yn, tramp-process-actions): Move let-binding of
`enable-recursive-minibuffers' up.
(tramp-handle-make-process, tramp-handle-write-region):
* lisp/net/tramp-adb.el (tramp-adb-handle-write-region)
(tramp-adb-handle-make-process):
* lisp/net/tramp-sh.el (tramp-sh-handle-make-process)
(tramp-sh-handle-write-region):
* lisp/net/tramp-smb.el (tramp-smb-handle-write-region):
* lisp/net/tramp-sshfs.el (tramp-sshfs-handle-write-region):
Use `string-or-null-p'.