html.c: add various strbuf and varadic helpers

This adds the fmtalloc helper, html_txtf, html_vtxtf, and html_attrf.

These takes a printf style format string like htmlf but escapes the
resulting string.  The html_vtxtf variant takes a va_list whereas
html_txtf is variadic.

Signed-off-by: John Keeping <john@keeping.me.uk>

html.h

@@ -1,7 +1,7 @@
 #ifndef HTML_H
 #define HTML_H
 
-#include <stddef.h>
+#include "cgit.h"
 
 extern void html_raw(const char *txt, size_t size);
 extern void html(const char *txt);
@@ -9,6 +9,15 @@ extern void html(const char *txt);
 __attribute__((format (printf,1,2)))
 extern void htmlf(const char *format,...);
 
+__attribute__((format (printf,1,2)))
+extern void html_txtf(const char *format,...);
+
+__attribute__((format (printf,1,0)))
+extern void html_vtxtf(const char *format, va_list ap);
+
+__attribute__((format (printf,1,2)))
+extern void html_attrf(const char *format,...);
+
 extern void html_status(int code, const char *msg, int more_headers);
 extern void html_txt(const char *txt);
 extern void html_ntxt(int len, const char *txt);