Use evbuffer for building S, W, L lists, also add an xmemdup helper

instead of misusing asprintf.

xmalloc.h

@@ -26,6 +26,7 @@ void	*xreallocarray(void *, size_t, size_t);
 void	*xrecallocarray(void *, size_t, size_t, size_t);
 char	*xstrdup(const char *);
 char	*xstrndup(const char *, size_t);
+char	*xmemdup(const void *, size_t);
 int	 xasprintf(char **, const char *, ...)
 		__attribute__((__format__ (printf, 2, 3)))
 		__attribute__((__nonnull__ (2)));