declare initial dot domains valid by fiat

This really, really sucks. But I am very aggressively not allowing
attribute values I define as invalid to exist on Cookie objects,
which prevents even temporarily checking if there is an initial dot.
Domain names with initial dot are NOT RFC valid and we shouldn't
produce them, but we might want to see them so they are "valid"
as far as that goes.
prevent .domain in Set-Cookie renders

Even if we are going to accept this junk in parses for users,
and as a matter of necessity also allow it to exist on Cookie objects,
and even if the User-Agent is obligated to ignore the dot anyway,
we should NOT be generating this RFC-invalid output by default.
don't allow domain dot to affect comparison

If you want to know if there is an initial domain dot,
then check the value yourself rather than comparing
to another cookie, there is no semantic difference between
cookie domains with a dot vs. without, and this is not just
the RFC but how User-Agents actually use the attribute
version bump