wsp: Improve multi-finger touchpad usage and allow more configurations

This patch allows scrolling with multiple fingers simultaneously, in
line with how wsp trackpads function on MacOS.

Two new tunables are added: hw.usb.wsp.max_finger_area and
hw.usb.wsp.max_double_tap_distance.

max_finger_area defines the maximum size which the driver registered an
object on trackpad as a finger.
Previously, this value was hardcoded as 1200, which was too low to
register thumb-clicks.

max_double_tap_distance defines the maximum distance between two
fingers which will register as a double-click.

Signed-off-by: Joshua Rogers <Joshua@Joshua.Hu>
Reviewed by: imp, wulf
Pull Request: https://github.com/freebsd/freebsd-src/pull/1365