Tighten restrictions on core <-> vendor socket comms

This futher restricts neverallows for sockets which may be exposed as
filesystem nodes. This is achieved by labelling all such sockets
created by core/non-vendor domains using the new coredomain_socket
attribute, and then adding neverallow rules targeting that attribute.

This has now effect on what domains are permitted to do. This only
changes neverallow rules.

Test: mmm system/sepolicy
Bug: 36577153
Change-Id: I633163cf67d60677c4725b754e01097dd5790aed