(table-cell-horizontal-chars): Renamed from table-cell-horizontal-char. Now a

(table-cell-horizontal-chars): Renamed from table-cell-horizontal-char.  Now a
string value instead of a character.  ?= is allowed for horizontal boundary as
well as ?-.
(table-command-remap-alist, table-command-list): Changed defconst to defvar
because the value is modified.
(table-insert, table-insert-row, table-insert-column, table-recognize)
(table-recognize-region, table-widen-cell, table-span-cell)
table-split-cell-vertically): Change due to table-cell-horizontal-chars.
(table--cell-horizontal-char-p): New function.
(table--generate-source-scan-lines, table-delete-row, table-delete-column)
(table--spacify-frame, table--find-row-column, table--probe-cell-left-up)
(table--probe-cell-right-bottom, table--probe-cell): Change due to
table-cell-horizontal-chars.
From David Abrahams <dave@boost-consulting.com>