Return separators from 'eshell-split-commands' directly when requested

Return separators from 'eshell-split-commands' directly when requested

This eliminates the need for using a dynamically-bound variable to
hold the list of separators.

* lisp/eshell/esh-cmd.el (eshell--sep-terms): Remove.
(eshell-split-commands): New function, adapted from
'eshell-separate-commands'.
(eshell-separate-commands): Make obsolete, and call
'eshell-split-commands'.
(eshell-parse-command, eshell-parse-pipeline): Use
'eshell-split-commands'.

* lisp/eshell/esh-arg.el (eshell-parse-delimiter): Update comment.