Move several completions from eshell to pcomplete (Bug#10585)

Move several completions from eshell to pcomplete (Bug#10585)

* lisp/eshell/em-unix.el (eshell-complete-hostname)
(pcomplete/ftp, pcomplete/ncftp, pcomplete/ping)
(pcomplete/rlogin, pcomplete/telnet, pcomplete/rsh):
Move from here...
* lisp/pcmpl-unix.el (pcmpl-unix-complete-hostname)
(pcomplete/ftp, pcomplete/ncftp, pcomplete/ping)
(pcomplete/rlogin, pcomplete/telnet, pcomplete/rsh):
...to here.  Make old names into aliases.

* lisp/eshell/esh-util.el (eshell-hosts-file)
(eshell-host-names, eshell-host-timestamp)
(eshell-read-hosts-file, eshell-read-hosts)
(eshell-read-host-names): Move from here...
* lisp/pcomplete.el (pcomplete-hosts-file)
(pcomplete--host-name-cache)
(pcomplete--host-name-cache-timestamp)
(pcomplete-read-hosts-file, pcomplete-read-hosts)
(pcomplete-read-host-names): ...to here.  Make old names into
aliases.

* lisp/eshell/em-unix.el (eshell-complete-host-reference): Update
caller.