libnetwork/osl: remove Interface Interface and other funny stories
### libnetwork/osl: add godoc to networkNamespace

Copying the descriptions from the Sandbox, Info, NeighborOptionSetter,
and IfaceOptionSetter interfaces that it implements.

### libnetwork: implement Controller.setupOSLSandbox

osl.NewSandbox() always returns a nil interface on Windows (and other non-Linux
platforms). This means that any code that these fields are always nil, and
any code using these fields must be considered Linux-only;

- libnetwork/Controller.defOsSbox
- libnetwork/Sandbox.osSbox

Ideally, these fields would live in Linux-only files, but they're referenced
in various platform-neutral parts of the code, so let's start with moving
the initialization code to Linux-only files.

### libnetwork: move more osl.Sandbox related code to Linux-only files

osl.NewSandbox() always returns a nil interface on Windows (and other non-Linux
platforms). This means that any code that these fields are always nil, and
any code using these fields must be considered Linux-only.

### libnetwork/osl: remove Sandbox.NeighborOptions() etc.

NeighborOptions() returned an NeighborOptionSetter interface, which
contained "methods" that returned functional options. Such a construct
could have made sense if the functional options returned would (e.g.)
be pre-propagated with information from the Sandbox (network namespace),
but none of that was the case.

There was only one implementation of NeighborOptionSetter (networkNamespace),
which happened to be the same as the only implementation of Sandbox, so
remove the interface as well, to help networkNamespace with its multi-personality
disorder.

This patch:

- removes Sandbox.LinkName() and makes it a regular function (WithLinkName)
- removes Sandbox.Family() and makes it a regular function (WithFamily)
- removes Sandbox.NeighborOptions().
- removes the NeighborOptionSetter interface

### libnetwork/osl: remove Sandbox.InterfaceOptions() etc.

InterfaceOptions() returned an IfaceOptionSetter interface, which contained
"methods" that returned functional options. Such a construct could have made
sense if the functional options returned would (e.g.) be pre-propagated with
information from the Sandbox (network namespace), but none of that was the case.

There was only one implementation of IfaceOptionSetter (networkNamespace),
which happened to be the same as the only implementation of Sandbox, so remove
the interface as well, to help networkNamespace with its multi-personality
disorder.

This patch:

- removes Sandbox.Bridge() and makes it a regular function (WithIsBridge)
- removes Sandbox.Master() and makes it a regular function (WithMaster)
- removes Sandbox.MacAddress() and makes it a regular function (WithMACAddress)
- removes Sandbox.Address() and makes it a regular function (WithIPv4Address)
- removes Sandbox.AddressIPv6() and makes it a regular function (WithIPv6Address)
- removes Sandbox.LinkLocalAddresses() and makes it a regular function (WithLinkLocalAddresses)
- removes Sandbox.Routes() and makes it a regular function (WithRoutes)
- removes Sandbox.InterfaceOptions().
- removes the IfaceOptionSetter interface.

Note that the IfaceOption signature was changes as well to allow returning
an error. This is not currently used, but will be used for some options
in the near future, so adding that in preparation.

### libnetwork/osl: remove Interface Interface

There's only one implementation; let's use that.
Also fixing a linting issue;

    libnetwork/osl/interface_linux.go:91:2: S1001: should use copy(to, from) instead of a loop (gosimple)
    for i, iface := range n.iFaces {
        ^

### libnetwork/osl: remove Interface.Master() as it's unused


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libnetwork/osl/interface_unsupported.go
//go:build !linux

package osl

type nwIface struct{}

//go:build !linux

package osl

type Interface struct{}

Need to remove all of this, because it looks like this is only needed because `libnetwork/osl` is imported in some windows files, but not actually doing anything.