Request: new request session flag to mark those files opened by FDT

This patch aims to fix a potential DDoS problem that can be caused
in the server quering repetitive non-existent resources.

When serving a static file, the core use Vhost FDT mechanism, but if
it sends a static error page it does a direct open(2). When closing
the resources for the same request it was just calling mk_vhost_close()
which did not clear properly the file descriptor.

This patch adds a new field on the struct session_request called 'fd_is_fdt',
which contains MK_TRUE or MK_FALSE depending of how fd_file was opened.

Thanks to Matthew Daley <mattd@bugfuzz.com> for report and troubleshoot this
problem.

Signed-off-by: Eduardo Silva <eduardo@monkey.io>
static mk_ptr_t *mk_request_set_default_page(char *title, mk_ptr_t message,
                                        char *signature)
{
    char *temp;
    mk_ptr_t *p;

    p = mk_mem_malloc(sizeof(mk_ptr_t));
    p->data = NULL;

    temp = mk_ptr_to_buf(message);
    mk_string_build(&p->data, &p->len,
                    MK_REQUEST_DEFAULT_PAGE, title, temp, signature);

    mk_mem_free(temp);

    return p;
}

static mk_ptr_t *mk_request_set_default_page(char *title, mk_ptr_t message,
                                        char *signature)
{
    char *temp;
    mk_ptr_t *p;

    p = mk_mem_malloc(sizeof(mk_ptr_t));
    p->data = NULL;

    temp = mk_ptr_to_buf(message);
    mk_string_build(&p->data, &p->len,
                    MK_REQUEST_DEFAULT_PAGE, title, temp, signature);

    mk_mem_free(temp);

    return p;
}
