Improve handling of processes on remote hosts.

Improve handling of processes on remote hosts.

* eshell/esh-util.el (eshell-path-env): New defvar.
(eshell-parse-colon-path): New defun.
(eshell-file-attributes): Use `eshell-parse-colon-path'.

* eshell/esh-ext.el (eshell-search-path): Use
`eshell-parse-colon-path'.
(eshell-remote-command): Remove argument HANDLER.
(eshell-external-command): Check for FTP remote connection.

* eshell/esh-proc.el (eshell-gather-process-output): Use
`file-truename', in order to start also symlinked files. Apply
`start-file-process' instead of `start-process'. Shorten `command'
to the local file name part.

* eshell/em-cmpl.el (eshell-complete-commands-list): Use
`eshell-parse-colon-path'.

* eshell/em-unix.el (eshell/du): Check for FTP remote connection.

* net/tramp.el (tramp-eshell-directory-change): New defun.  Add it
to `eshell-directory-change-hook'.