[emacs-w3m:13653] [FEATURE REQUEST] Remembering most recent save-path

Hi, Katsumi-san.

I was originally preparing this as a simple pull request, but then I
noticed something to make me feel I should ask a question first on how
you would want to proceed.

The idea is pretty simple, and exists in pretty much any other browser:
The default save directory should be remembered from the last time a
save was performed during the current session. For my
personally-modified version of the project, it seemed simple at first to insert

  (setq w3m-save-buffer-directory (file-name-directory name))

in two places, but then I saw that there seems to be an unnecessary
duplication of the variables `w3m-save-buffer-directory' and
`w3m-default-save-directory':

w3m-save.el:33:(defcustom w3m-save-buffer-directory (expand-file-name
w3m-save.el:57:Save the current buffer as `w3m-save-buffer-directory'/NAME.html,
w3m-save.el:59:folder `w3m-save-buffer-directory'/NAME-files/. Use of
w3m-save.el:60:`w3m-save-buffer-directory' may be over-ridden by including a
w3m-save.el:87:	   (file-name-as-directory w3m-save-buffer-directory)
w3m-save.el:91:  (setq w3m-save-buffer-directory (file-name-directory name))
w3m-save.el:114:      (setq name (expand-file-name name w3m-save-buffer-directory)))

w3m.el:784:(defcustom w3m-default-save-directory
w3m.el:4813:  (setq dir (file-name-as-directory (or dir w3m-default-save-directory)))
w3m.el:4817:	(setq w3m-default-save-directory
w3m.el:4818:	      (or (file-name-directory file) w3m-default-save-directory))
w3m.el:4819:      (setq w3m-default-save-directory file)
w3m.el:5833:		  (eval (car (get 'w3m-default-save-directory 'standard-value))
w3m.el:5835:		(error w3m-default-save-directory))))
w3m.el:5845:			w3m-default-save-directory basename)
w3m.el:9311:			       w3m-default-save-directory))
w3m-save.el:35:				      w3m-default-save-directory)

Someone at some point clearly did think it was a good idea to have two
separate variables (see the defcustom at w3m-save.el:33), but maybe that
decision should be revisited? It seems to me to be only an invitation
for confusion. If you agree, then the next question would be which to
keep and which to discard, and how to do it so the change won't break
any users' current and long-standing set-up.

Let me know what you think and I can prepare a pull request accordingly,
or if its not worth the bother to you and you want to make the commit
yourself, that's also fine of course.

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