Use "/local:" prefix in Eshell to run local commands when cwd is remote

Use "/local:" prefix in Eshell to run local commands when cwd is remote

* lisp/eshell/esh-ext.el (eshell-explicit-remote-commands)
(eshell-explicit-command): Update docstrings.
(eshell--local-prefix): New constant.
(eshell-handle-remote-command): Remove.
(eshell-quoted-file-command): New function...
(eshell-ext-initialize): ... add it as a hook.
(eshell-remote-command): Support running commands on localhost.
(eshell-connection-local-command): Rename from
'eshell-external-command'.
(eshell-external-command): New implementation calling
'eshell-remote-command' or 'eshell-connection-local-command' as
appropriate.

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

* doc/misc/eshell.texi (Remote Access): Update documentation.

* etc/NEWS: Update announcement.