* tramp.el (tramp-error): VEC-OR-PROC can be nil.
(tramp-action-password): Clear password cache if needed.
(tramp-read-passwd): Do not clear password cache.
* tramp-sh.el (tramp-sh-handle-vc-registered): Run first pass only
when `remote-file-name-inhibit-cache' is nil.
(tramp-sh-file-name-handler): Use `tramp-error'.  Simplify code.
* tramp-tests.el (tramp-copy-size-limit): Declare.
(tramp-test10-write-region): Extend for out-of-band copy.
(tramp-test31-asynchronous-requests): New test.