bug#41537: 26.3; window-body-width wrong return value when display-line-numbers-mode is on

["Matsievskiy S.V." <seregaxvm.main@gmail.com>]

`window-body-width` and `window-width` return wrong numbers when
`display-line-numbers-mode` is on.

For example:
```emacs-lisp
(message (format "Width without line numbers: %d" (window-body-width)))
(display-line-numbers-mode 1)
(message (format "Width with line numbers: %d" (window-body-width)))
```

return these lines:
```
Width without line numbers: 209
Width with line numbers: 209
```

This is obviously wrong since line numbers take some window space.
This breaks some packages that rely on `window-body-width` and
`window-width` to align strings. 


In GNU Emacs 26.3 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.20)
 of 2020-05-17, modified by Debian built on x86-csail-01
Windowing system distributor 'The X.Org Foundation', version
11.0.12008000
System Description:     Debian GNU/Linux bullseye/sid

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Mark set
Width without line numbers: 209
Width with line numbers: 209

Configured using:
 'configure --build x86_64-linux-gnu --prefix=/usr
 --sharedstatedir=/var/lib --libexecdir=/usr/lib
 --localstatedir=/var/lib --infodir=/usr/share/info
 --mandir=/usr/share/man --enable-libsystemd --with-pop=yes
 --enable-locallisppath=/etc/emacs:/usr/local/share/emacs/26.3/site-
lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/26.3/site-
lisp:/usr/share/emacs/site-lisp
 --with-sound=alsa --without-gconf --with-mailutils --build
 x86_64-linux-gnu --prefix=/usr --sharedstatedir=/var/lib
 --libexecdir=/usr/lib --localstatedir=/var/lib
 --infodir=/usr/share/info --mandir=/usr/share/man --enable-libsystemd
 --with-pop=yes
 --enable-locallisppath=/etc/emacs:/usr/local/share/emacs/26.3/site-
lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/26.3/site-
lisp:/usr/share/emacs/site-lisp
 --with-sound=alsa --without-gconf --with-mailutils --with-x=yes
 --with-x-toolkit=gtk3 --with-toolkit-scroll-bars 'CFLAGS=-g -O2
 -fdebug-prefix-map=/build/emacs-mHAik2/emacs-26.3+1=. -fstack-
protector-strong
 -Wformat -Werror=format-security -Wall' 'CPPFLAGS=-Wdate-time
 -D_FORTIFY_SOURCE=2' LDFLAGS=-Wl,-z,relro'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS GLIB
NOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM THREADS LIBSYSTEMD LCMS2

Important settings:
  value of $LANG: en_GB.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  display-line-numbers-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny seq byte-opt gv
bytecomp byte-compile cconv dired dired-loaddefs format-spec rfc822 mml
mml-sec password-cache epa derived epg epg-config gnus-util rmail
rmail-loaddefs mm-decode mm-bodies mm-encode mail-parse rfc2231
mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045 ietf-drums
mm-util mail-prsvr mail-utils thingatpt help-fns radix-tree help-mode
easymenu cl-loaddefs cl-lib misearch multi-isearch display-line-numbers
cus-start cus-load elec-pair time-date mule-util tooltip eldoc electric
uniquify ediff-hook vc-hooks lisp-float-type mwheel term/x-win x-win
term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode elisp-mode lisp-mode
prog-mode register page menu-bar rfn-eshadow isearch timer select
scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame cl-generic cham georgian utf-8-lang misc-lang
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chinese composite charscript charprop case-table epa-hook jka-cmpr-hook
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base64 format env code-pages mule custom widget hashtable-print-
readable
backquote threads dbusbind inotify lcms2 dynamic-setting
system-font-setting font-render-setting move-toolbar gtk x-toolkit x
multi-tty make-network-process emacs)

Memory information:
((conses 16 109750 7626)
 (symbols 48 21505 1)
 (miscs 40 68 215)
 (strings 32 32568 1315)
 (string-bytes 1 804681)
 (vectors 16 14249)
 (vector-slots 8 504282 12068)
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 (buffers 992 13))