esh-opt.el: Fix improper parsing of first argument (Bug#28323)

esh-opt.el: Fix improper parsing of first argument (Bug#28323)

Examples of broken behavior:

    sudo -u root whoami
    Outputs: -u
    ls -I '*.txt' /dev/null
    Errors with: *.txt: No such file or directory

* lisp/eshell/esh-opt.el (eshell--process-args): Refactor usage of
args to eshell--args, as we rely on modifications from
eshell--process-option and vice versa.  These modifications were not
being propogated in the (if (= ai 0)) case, since popping the first
element of a list doesn't destructively modify the underlying list
object.