LibCore+LibIPC: Move SystemServerTakeover.{h,cpp} to LibCore

This functionality is required by Core::LocalServer and LibIPC depends
on LibCore.

take_over_accepted_socket_from_system_server has also been renamed to
take_over_socket_from_system_server as the socket need not be accepted
before taking it over. :^)

Userland/Libraries/LibIPC/CMakeLists.txt

@@ -4,7 +4,6 @@ set(SOURCES
     Encoder.cpp
     Message.cpp
     Stub.cpp
-    SystemServerTakeover.cpp
 )
 
 serenity_lib(LibIPC ipc)