Follow POSIX/GNU argument conventions for 'eshell-eval-using-options'

Follow POSIX/GNU argument conventions for 'eshell-eval-using-options'

* lisp/eshell/esh-opt.el (eshell--split-switch): New function.
(eshell-set-option): Allow setting a supplied value instead of always
consuming from 'eshell--args'.
(eshell--process-option): Support consuming option values specified as
a single token.
(eshell--process-args): For short options, pass full switch token to
'eshell--process-option'.

* test/lisp/eshell/esh-opt-tests.el (esh-opt-process-args-test): Fix
test.
(test-eshell-eval-using-options): Add tests for various types of
options.

* doc/misc/eshell.texi (Defining new built-in commands): New
subsection, describe how to use 'eshell-eval-using-options'.

* etc/NEWS: Announce the change.