We have a new, improved string type coming up in AK (OOM aware, no null
state), and while it's going to use UTF-8, the name UTF8String is a
mouthful - so let's free up the String name by renaming the existing
class.
Making the old one have an annoying name will hopefully also help with
quick adoption :^)
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. :^)