bug#28945: 25.2; desktop auto save timer does not work

[Peter Neidhardt <pe.neidhardt@googlemail.com>]

Recipe:

- Add (desktop-save-mode) to your init.el.

- Start Emacs.  Check the value of `window-configuration-change-hook':
the _global_ value should contain `desktop-auto-save-set-timer'.

- Save the desktop.

- Quit Emacs.

- Restart Emacs.

- Check `window-configuration-change-hook': the timer is gone.

I've narrowed down the issue to `desktop-read':

	;; ...
	(setq desktop-autosave-was-enabled
	  (memq 'desktop-auto-save-set-timer 'window-configuration-change-hook))
	(desktop-auto-save-disable)
	;; ...
	(if desktop-autosave-was-enabled (desktop-auto-save-enable))

The check is done on the _local_ value of
`window-configuration-change-hook', thus `desktop-auto-save-enable' will
never be re-run.

The fix is as simple as

	(setq desktop-autosave-was-enabled
	  (memq 'desktop-auto-save-set-timer (default-toplevel-value 'window-configuration-change-hook)))



I've been annoyed by this issue for years but before today I could not
find the logic behind the issue and did not know what to report.  The
issue effectively makes desktop-mode half-useless since desktop won't be
saved when Emacs gets killed too abruptly (crashes, power outage...).

Did nobody notice this before or is it that nobody could figure out the
logic like me?



While we are at it, we might change the documentation and value of `desktop-auto-save-timeout':

	Number of seconds idle time before auto-save of the desktop.
	The idle timer activates auto-saving only when window configuration changes.

This is terribly confusing, isn't it?  One might wonder whether it's a
timer that saves every 30 seconds or the configuration change.

The actual logic is as follow:

- On every window configuration change, the timer is _reset_.

- When idle for `desktop-auto-save-timeout', the desktop is saved.  The
  timer is run only _once_.

In practice, this means that the user actually be idle for 30 seconds or
else it won't get saved at all.  Does this happen a lot in practice?
I'm don't think so.  Considering the timer runs only once per window
conf change, it does not run very often, in particular it runs only once
when nothing is happening.

So I think it's wiser to set the default to something much lower, say 5
or 10.



Lastly, a minor nit: desktop.el adds a lambda to `after-init-hook'; can
we turn this into a named function?



In GNU Emacs 25.2.1 (x86_64-pc-linux-gnu, GTK+ Version 3.22.16)
 of 2017-09-02 built on dhiov23k
Windowing system distributor 'The X.Org Foundation', version 11.0.11905000
System Description:	Gentoo Base System release 2.4.1

Configured using:
 'configure --prefix=/usr --build=x86_64-pc-linux-gnu
 --host=x86_64-pc-linux-gnu --mandir=/usr/share/man
 --infodir=/usr/share/info --datadir=/usr/share --sysconfdir=/etc
 --localstatedir=/var/lib --disable-dependency-tracking
 --disable-silent-rules --docdir=/usr/share/doc/emacs-25.2
 --htmldir=/usr/share/doc/emacs-25.2/html --libdir=/usr/lib64
 --program-suffix=-emacs-25 --infodir=/usr/share/info/emacs-25
 --localstatedir=/var
 --enable-locallisppath=/etc/emacs:/usr/share/emacs/site-lisp
 --with-gameuser=:gamestat --without-compress-install
 --with-file-notification=inotify --enable-acl --without-dbus
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 --with-gif --with-jpeg --with-png --with-rsvg --with-tiff --with-xpm
 --with-imagemagick --with-xft --without-cairo --without-libotf
 --without-m17n-flt --with-x-toolkit=gtk3 --without-xwidgets
 GENTOO_PACKAGE=app-editors/emacs-25.2 'CFLAGS=-march=ivybridge -O2
 -pipe' CPPFLAGS= 'LDFLAGS=-Wl,-O1 -Wl,--as-needed''

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND NOTIFY ACL GNUTLS LIBXML2
FREETYPE XFT ZLIB GTK3 X11

Important settings:
  value of $LANG: en_US.utf8
  locale-coding-system: utf-8-unix

Major mode: Debbugs

Minor modes in effect:
  semantic-minor-modes-format: ((:eval (if (or semantic-highlight-edits-mode semantic-show-unmatched-syntax-mode)  S)))
  recentf-mode: t
  pdf-occur-global-minor-mode: t
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  shell-dirtrack-mode: t
  evil-mode: t
  evil-local-mode: t
  show-paren-mode: t
  savehist-mode: t
  save-place-mode: t
  desktop-save-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  file-name-shadow-mode: t
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  buffer-read-only: t
  size-indication-mode: t
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