Wez Furlong's patch to tidy up full-screen mode: make it

Wez Furlong's patch to tidy up full-screen mode: make it
multi-monitor aware and make the scrollbar separately configurable
in and out of full-screen mode. Also (not Wez's patch, this bit) fix
the case where the user reconfigures _while_ the window is
full-screen, and disables full-screening. (In this case the window
should return gracefully to normal, rather than losing all its title
bars and getting confused.)

git-svn-id: svn://svn.tartarus.org/sgt/putty@1310 cda61777-01e9-0310-a592-d414129be87e