The sendfile filter fails unless Dir is also used

The sendfile needs the MAX_SEND_BUFFER variable, but it is initialized only in Dir_create, so in a server that uses sendfile filters only, any request fail as the buffer has a size of 0 bytes.

I'm not sure if this patch is the most clever way to fix the issue, as it makes the sendfile filter depend on Dir. A solution would be to move MAX_SEND_BUFFER to io.c because currently Dir and IO depends on each other.