Fix @multitable handling in texi2pod.pl

Fix @multitable handling in texi2pod.pl

While trying out Dennis's Armv8.6-A patch, I noticed that texi2pod.pl
didn't handle the new @multitable correctly.  There were two problems:

(1) @multitables nested in other @tables inherited the @item type from
    the enclosing @table.  Since the new @multitable is in a @table @samp,
    we applied @samp markup to the @multitable @items.  This in turn
    meant that it captured the @tab separator in the @item markup.

    Fixed by pushing an empty item code onto the stack.

(2) We didn't handle @headitem.  Fixed by enclosing it in italics,
    like we do for section headings.  This causes it to be underlined
    in the man output.

2019-12-07  Richard Sandiford  <richard.sandiford@arm.com>

contrib/
	* texi2pod.pl: Handle @headitems in @multitables, printing them
	in italics.  Push an empty item code onto the stack.

From-SVN: r279074