Sync up to version 19.

Sync up to version 19.
(savehist-mode): New minor mode.
(savehist-file): Use ~/.emacs.d or ~/.xemacs if available.
(savehist-length): Remove (use history-length instead).
(savehist-file-modes): Rename from savehist-modes.
(savehist-save-hook, savehist-loaded): New vars.
(savehist-load): Use savehist-mode.  Try to smooth up transition from
old format to new format.
(savehist-install): Allow savehist-autosave-interval to be nil.
(savehist-save): Run the new hook.  Be more careful to only trim the
history variables.
(savehist-trim-history): New fun.  Replaces savehist-process-for-saving.
(savehist-printable): Print into a buffer rather than char-by-char.