testutil/daemon uses a generic unix implementation that assumes that
the host OS supports cgroups & network namespaces, which is not the
case for FreeBSD.
This change adds a FreeBSD-specific implementation for
`testutil/daemon`, namely for `cleanupNetworkNamespace` and
`CgroupNamespace` functions.
Signed-off-by: Artem Khramov <akhramov@pm.me>
