/etc/mnttab is required by libzfs to get running properly, so let's
create it as an empty file.

ryao from zfsonlinux and openzfs told me that an empty /etc/mnttab is used
on Linux. Also reading the libzfs code shows that /etc/mnttab mostly used for
management of the file itself, nothing that would prevent some libzfs
functionality from working.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>