Improve pp-emacs-lisp-code backquote form printing

Improve pp-emacs-lisp-code backquote form printing

* lisp/emacs-lisp/pp.el (pp--quoted-or-unquoted-form-p): New helper
function.
(pp--insert-lisp): Take care of quoted, backquoted and
unquoted expressions; print using an recursive call.
(pp--format-list): Exclude more cases from printing as a function call
by default.  Print lists whose second-last element is an (un)quoting
symbol using dotted list syntax; e.g. (a b . ,c) instead of (a b \, c).