Fixes #409.  Improve select accelerator.  Fix bottom, top lags.

This pull request addresses issue 409.  It explicitly fixes the second part of 409, which states "you can't scroll to the most recent line" when selecting causes a scrolldown.  The same problem occurs when selecting causes a scrollup.  Both problems are fixed by this pull request.

The first part of issue 409 is not reproducible, and until it becomes so, I will not address it.  If you can reproduce it with this fix (or even without), please open a new issue and supply files with it:  (1) hexchat.conf and (2) at least one file from scrollback/<network>/<channel>.txt and I will try myself to reproduce the problem on that network with that channel and scrollback.  I don't run windows so please try reproducing on a linux distro, preferably Fedora.