Currently you can only mmap the entire framebuffer.
Using this when starting up the WindowServer gets us yet another step
closer towards it moving into userspace. :^)
This required a fair bit of plumbing. The CharacterDevice::close() virtual
will now be closed by ~FileDescriptor(), allowing device implementations to
do custom cleanup at that point.
One big problem remains: if the master PTY is closed before the slave PTY,
we go into crashy land.
