control: enforce registration policies

[kormat]

• an up-segment registration is only allowed from the local AS, and can only happen in a non-core AS
• a down-segment registration is only allowed from the local ISD, and can only happen in a core AS
• a core-segment registration is only allowed from the local AS, and can only happen in a core AS

Update: since the "monolithization" of the control service, up and core segment registration no longer exist as explicit RPCs.

[jiceatscion]

The check is indeed still missing. It seems that it could go about here:

scion/control/segreg/grpc/registration_server.go

Line 75 in b01d019

Note that in the preceding line, labels.Type is derived from the registration request and not from the segment itself. We should get that info from the segment and the invoker shouldn't need to tell us (and we shouldn't believe it).