Support the button element as defined in HTML 4.01

Patch from upstream, [w3m-dev 04411] on 2010-09-17, to support the
button element.  It is discussed upstream and incomplete, but enough
to login Launchpad.

html.h

@@ -214,21 +214,22 @@ typedef struct {
 #define HTML_BIG        122
 #define HTML_N_BIG      123
 #define HTML_BUTTON     124
-#define HTML_FIELDSET   125
-#define HTML_N_FIELDSET 126
-#define HTML_IFRAME     127
-#define HTML_LABEL      128
-#define HTML_N_LABEL    129
-#define HTML_LEGEND     130
-#define HTML_N_LEGEND   131
-#define HTML_NOSCRIPT   132
-#define HTML_N_NOSCRIPT 133
-#define HTML_OBJECT     134
-#define HTML_OPTGROUP   135
-#define HTML_N_OPTGROUP 136
-#define HTML_PARAM      137
-#define HTML_SMALL      138
-#define HTML_N_SMALL    139
+#define HTML_N_BUTTON   125
+#define HTML_FIELDSET   126
+#define HTML_N_FIELDSET 127
+#define HTML_IFRAME     128
+#define HTML_LABEL      129
+#define HTML_N_LABEL    130
+#define HTML_LEGEND     131
+#define HTML_N_LEGEND   132
+#define HTML_NOSCRIPT   133
+#define HTML_N_NOSCRIPT 134
+#define HTML_OBJECT     135
+#define HTML_OPTGROUP   136
+#define HTML_N_OPTGROUP 137
+#define HTML_PARAM      138
+#define HTML_SMALL      139
+#define HTML_N_SMALL    140
 
    /* pseudo tag */
 #define HTML_SELECT_INT     160