WM_SIZE/SIZE_MAXIMIZED can show up even during an interactive resize,

WM_SIZE/SIZE_MAXIMIZED can show up even during an interactive resize,
so we should ensure we treat it the same way as other WM_SIZEs that
show up during that time: set the width and height in conf, and set
the flag to have that width and height enacted on WM_EXITSIZEMOVE.

Fixes a bug in which dragging a PuTTY window directly from the Win7
snapped-to-half-screen position to the snapped-to-maximised state
would leave the terminal in the pre-snapped size.

[originally from svn r9404]