change log for startup.el

[Richard Stallman]

        * startup.el: Backport startup screen related changes from the trunk.

That's not an adequate change log, because it does not describe the
changes.  Saying where they came from is useful but it is no
substitute for saying what they are.

Would you please copy the change log entries for these changes from
the trunk?  It also might be good to combine them and simplify them,
since that might make them much shorter and more comprehensible.

Please ack when it is done.