Richard B's patch to support X cut buffers as well as ordinary

Richard B's patch to support X cut buffers as well as ordinary
selections, meaning that (a) a pterm can leave copied text in the
cut buffer after it terminates so that applications can pick it up
even though it isn't still around to deliver the selection in
person, and (b) pterm can pick up things left in this way by other
apps.
Downside is that all of this only happens in ISO8859-1, because X is
weird like that.

git-svn-id: svn://svn.tartarus.org/sgt/putty@3409 cda61777-01e9-0310-a592-d414129be87e