Improve Synaptics support for newer touchpads.

Enable two finger scrolling by default and disable the edge scrolling if
the touchpad has no physical zone for it.  Disable directional scrolling
by default to avoid using extended buttons as scroll buttons.

Add support for ClickPad.  On Lenovo laptops, this is the button
reported when one presses the touchpad.

While there, fix a problem where the extended buttons were not reporting
the button release event correctly: we need to save the state of the
buttons and report it to sysmouse until we receive a packet from the
touchpad indicating the button has been released.  This makes it
possible to use an extended button to resize a window.  On Lenovo
laptops, the major buttons are actually reported as extended buttons.