Avoid the obsolete `assoc' package.

Avoid the obsolete `assoc' package.
* lisp/speedbar.el (speedbar-refresh): Avoid adelete.
(speedbar-file-lists): Simplify and avoid aput.
* lisp/man.el (Man--sections, Man--refpages): New vars, replacing
Man-sections-alist and Man-refpages-alist.
(Man-build-section-alist, Man-build-references-alist):
Use them; avoid aput.
(Man--last-section, Man--last-refpage): New vars.
(Man-follow-manual-reference): Use them.
Use the `default' arg of completing-read.
(Man-goto-section): Idem.  Move prompt to the `interactive' spec.
* lisp/gnus/auth-source.el (auth-source--aput-1, auth-source--aput)
(auth-source--aget): New functions and macros.
Use them instead of aput/aget.