bug#28943: 25.2; desktop-kill should not be in kill-emacs-hook

[Pierre Neidhardt <ambrevar@gmail.com>]

As per the documentation of `kill-emacs-hook':

> functions placed on this hook should not
> expect to be able to interact with the user.  To ask for confirmation,
> see ‘kill-emacs-query-functions’ instead.

In the meanwhile, `desktop-kill':

> If the desktop should be saved and ‘desktop-dirname’
> is nil, ask the user where to save the desktop.

I suggest either we move `desktop-kill' to `kill-emacs-query-functions'
or we define two functions, one with user interaction, the other one
unattended.



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
 --without-modules --without-gpm --without-hesiod --without-kerberos
 --without-kerberos5 --with-xml2 --without-selinux --with-gnutls
 --without-wide-int --with-zlib --with-sound=alsa --with-x --without-ns
 --without-gconf --without-gsettings --without-toolkit-scroll-bars
 --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