Fix Bug#23614.

Fix Bug#23614.

* lisp/net/tramp-sh.el (tramp-maybe-open-connection):
Let-bind `process-coding-system-alist' in order to suppress
the value for "cmdproxy".

* lisp/net/tramp.el (tramp-encoding-shell):
`w32-shell-name' is a function.
(tramp-encoding-command-switch)
(tramp-encoding-command-interactive): Use `w32-shell-dos-semantics'.

* test/lisp/net/tramp-tests.el (tramp-test24-file-name-completion):
Suppress some tests on MS Windows.  (Bug#23614)