bug#23456: 25.0.93; Explicitely bind <backtab> to 'table-backward-cell'

[Tino Calancha <f92capac@gmail.com>]

The issue only happen wen running Emacs without graphics (i.e. inside 
terminal)

emacs -Q -nw --eval="(require 'table)"
;; Notice the option '-nw'
;; Insert following text in *scratch* buffer:

  +-------------------------------+----------------------------------+
  |`table-insert'                 |Insert a table consisting of grid |
  |                               |of cells by specifying the number |
  |                               |of COLUMNS, number of ROWS, cell  |
  |                               |WIDTH and cell HEIGHT.            |
  +-------------------------------+----------------------------------+
  |`table-insert-row'             |Insert row(s) of cells before the |
  |                               |current row that matches the      |
  |                               |current row structure.            |
  +-------------------------------+----------------------------------+
  |`table-insert-column'          |Insert column(s) of cells before  |
  |                               |the current column that matches   |
  |                               |the current column structure.     |
  +-------------------------------+----------------------------------+
M-x table-recognize RET
;; Shift + TAB shows error:
;; <backtab> is undefined
;; When running Emacs with graphics the binding already exists.


In GNU Emacs 25.0.93.9 (x86_64-pc-linux-gnu, GTK+ Version 2.24.30)
  of 2016-05-05 built on calancha-pc
Repository revision: 1a4127dbd625ea64f535b3bd09844a99161290a6
Windowing system distributor 'The X.Org Foundation', version 11.0.11803000
System Description:	Debian GNU/Linux unstable (sid)

;; Suggested patch:

diff --git a/lisp/textmodes/table.el b/lisp/textmodes/table.el
index 653db83..49411f4 100644
--- a/lisp/textmodes/table.el
+++ b/lisp/textmodes/table.el
@@ -936,6 +936,7 @@ table-cell-bindings
      ([(shift backtab)]	. table-backward-cell) ; for HPUX console keyboard
      ([(shift iso-lefttab)]    . table-backward-cell) ; shift-tab on a microsoft natural keyboard and redhat linux
      ([(shift tab)]	. table-backward-cell)
+    ([backtab]          . table-backward-cell) ; for terminals (e.g. xterm)
      ([return]		. *table--cell-newline)
      ([(control m)]	. *table--cell-newline)
      ([(control j)]	. *table--cell-newline-and-indent)