Use BUF_strlcpy() instead of strcpy().

Use BUF_strlcat() instead of strcat().
Use BIO_snprintf() instead of sprintf().
In some cases, keep better track of buffer lengths.
This is part of a large change submitted by Markus Friedl <markus@openbsd.org>

apps/engine.c

@@ -122,8 +122,8 @@ static int append_buf(char **buf, const char *s, int *size, int step)
 		return 0;
 
 	if (**buf != '\0')
-		strcat(*buf, ", ");
-	strcat(*buf, s);
+		BUF_strlcat(*buf, ", ", *size);
+	BUF_strlcat(*buf, s, *size);
 
 	return 1;
 	}