Add XSECURELOCK_FORCE_GRAB - a mode to try really hard to get the grabs.

XSECURELOCK_FORCE_GRAB=1: locate all client windows, hide them, grab,
unhide them. Will very likely rearrange them, but otherwise works with
most WMs.

XSECURELOCK_FORCE_GRAB=2: locate all windows below the root window, hide
them, grab, unhide them. Will solve some cases =1 does not, but probably
breaks most WMs.