Avoid the use of the old (eql VAL) specializer, preferring the new
(eql EXP) form.
* lisp/emacs-lisp/multisession.el (multisession-backend-value)
multisession--backend-set-value, multisession--backend-values)
multisession--backend-delete, multisession-backend-value)
multisession--backend-set-value, multisession--backend-values)
multisession--backend-delete): Quote the symbol passed to `eql` specializer.
* lisp/emacs-lisp/multisession.el: Don't require url -- this pulls
in gnus-util, which requires rmail, which depends on
rmail-loaddefs being generated, which our Makefiles doesn't guarantee.
