Never use the value attribute of <button> to encode anything we care

Never use the value attribute of <button> to encode anything we care
about, in particular 'action' arguments to the CGI.  This works around
a bug in Internet Explorer.

At the time of writing, the commentary at
http://msdn2.microsoft.com/en-gb/library/ms535211(VS.85).aspx mentions
the problem.  (Alternatively, MSDN Library -> Web Development -> HTML
and CSS -> HTML and DHTML Reference -> Objects -> button.)