Add a general way to request an immediate top-level callback.

Add a general way to request an immediate top-level callback.

This is a little like schedule_timer, in that the callback you provide
will be run from the top-level message loop of whatever application
you're in; but unlike the timer mechanism, it will happen
_immediately_.

The aim is to provide a general way to avoid re-entrance of code, in
cases where just _doing_ the thing you want done is liable to trigger
a confusing recursive call to the function in which you came to the
decision to do it; instead, you just request a top-level callback at
the message loop's earliest convenience, and do it then.

git-svn-id: http://svn.tartarus.org/sgt/putty@10019 cda61777-01e9-0310-a592-d414129be87e