bug#31348: 26.1; savehist-save errors if savehist-file directory missing

[Allen Li]

savehist-save does not create output directory for savehist-file, e.g.:

savehist-save: Opening output file: no such file or directory,
/home/bob/some/new/cache/dir/history

This is in contrast with other examples, such as
filesets-menu-cache-file, which offers to create the directory if it
does not exist.

This is annoying because the error stops C-x C-c from being able to
exit Emacs.

In GNU Emacs 26.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 2.24.31)
  of 2018-04-11 built on sharanohiar.mtv.corp.google.com
Repository revision: 92e0fd880c2d9689b89e13d317356c3ad00c7325
Windowing system distributor 'The X.Org Foundation', version 11.0.11906000
System Description: Debian GNU/Linux rodete (upgraded from: Ubuntu 14.04
LTS)