Correctly compute mouse.Msec time stamp

[fhs]

The time stamp should be in milliseconds, so take system time in
nanoseconds, convert to milliseconds and take the lower 32-bits.

This makes Edwood correctly detect double-clicks.
If the difference between consecutive time stamps is less
than 500 milliseconds, Edwood interprets it as a double-click.

[ktye]

I don't remember, if there are double click events in duit, that could be broken.
But if this is what libdraw expects, we should use it.

For the record: there is also shiny/gesture that detect double clicks (and long clicks, ...).