Add 'server-eval-args-left' to server.el

Add 'server-eval-args-left' to server.el

Passing arbitrary arguments to functions through
"emacsclient --eval" sometimes requires complicated escaping
to avoid them being parsed as Lisp (as seen in
emacsclient-mail.desktop before this change).

The new variable 'server-eval-args-left' allows access to the
arguments before they are parsed as Lisp.  By removing
arguments from the variable before they're parsed, a snippet
of Lisp can consume arguments, as in emacsclient-mail.desktop.

org-protocol might be able to use this as well, which might allow it
to drop its current advice on server-visit-files.

* etc/emacsclient-mail.desktop: Use 'server-eval-args-left'.
* lisp/server.el (server-eval-args-left): New variable.
(server-process-filter, server-execute): Make '-eval' arguments
available through 'server-eval-args-left'.
* lisp/startup.el (argv): Mention 'server-eval-args-left' in
docstring.
* etc/NEWS: Announce 'server-eval-args-left'.
* doc/emacs/misc.texi (emacsclient Options): Document
'server-eval-args-left'.  (Bug#65902)