Add support for explicitly-remote commands in Eshell

Add support for explicitly-remote commands in Eshell

* lisp/files.el (file-remote-p):
* doc/lispref/files.texi (Magic File Names): Document 'never' for
CONNECTED argument.

* lisp/net/tramp.el (tramp-handle-file-remote-p): Handle CONNECTED
value of 'never'.

* lisp/eshell/esh-ext.el (eshell-explicit-remote-commands): New
option.
(eshell-ext-initialize): Apply 'eshell-handle-remote-command' when
requested.
(eshell-handle-remote-command): New function.
(eshell-remote-command): Reimplement this function and dispatch to
'eshell-external-command', which can handle remote processes on its
own.

* test/lisp/eshell/esh-ext-tests.el
(esh-ext-test/explicitly-remote-command)
(esh-ext-test/explicitly-local-command): New tests.

* doc/misc/eshell.texi (Remote Access): Document explicitly-remote
commands.

* etc/NEWS: Announce this change.