bug#30452: 25.3; tabulated-list-mode-map should inherit from special-mode-map

[Stephen Jung]

tabulated-list-mode inherits from special-mode, so
tabulated-list-mode-map should probably also inherit from
special-mode-map. However, it actually copies special-mode-map in its
definition 
(https://git.savannah.gnu.org/cgit/emacs.git/tree/lisp/emacs-lisp/tabulated-list.el?h=emacs-25.3#n152).

This appears to be the way that the code was written when
tabulated-list-mode was first created. I haven't read the mailing list
archives, so I'm not sure if there was an explanation of why it was
done this way, and there isn't a comment providing details on this
approach. Therefore, I think it should just be making a sparse keymap
here and inheriting from its parent, like most modes do.

In GNU Emacs 25.3.1 (x86_64-pc-linux-gnu, GTK+ Version 3.22.26)
 of 2018-02-08 built on bisson
Windowing system distributor 'The X.Org Foundation', version 11.0.11906000
Configured using:
 'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
 --localstatedir=/var --with-x-toolkit=gtk3 --with-xft --with-modules
 'CFLAGS=-march=x86-64 -mtune=generic -O2 -pipe -fstack-protector-strong
 -fno-plt' CPPFLAGS=-D_FORTIFY_SOURCE=2
 LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GCONF GSETTINGS
NOTIFY ACL GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 MODULES

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

-- 
Stephen