[developers] type addenda and :< in TDL

[Ann Copestake]

from my perspective, it's there for backward compatability, but I'm
not keen to get rid of it since it'll cause hassle for no good reason.
Presumably you will be changing some :< cases to :=, but I don't want
to force everyone to change all of them.  You might be able to produce
a patch to tdltypeinput.lsp so it doesn't complain if you try and
augment a :< type, but that shouldn't be checked in, I feel.

Ann