MFC r314467,r328027: Add hw.usb.wsp.enable_single_tap_clicks sysctl

MFC r314467 (imp): Make wsp process a single touchpad tap and interpret it
as a left-click event. It can be disabled by setting the new
hw.usb.wsp.enable_single_tap_clicks sysctl to 0.

MFC r328027: wsp(4): Update to reflect new sysctl from r314467

r314467 introduced hw.usb.wsp.enable_single_tap_clicks to enable/disable
single-tap left click behavior. Update the man page to reflect the new
sysctl.

PR: 196624