pmsystem logics section: random values

Hi,

it had been asked a while back, but I never added it to the
docs or mentioned it... there is a system variable called
"random", which each logics cycle generates a new value
between 0 and 1000.

You can use it for all sorts of things, such as random
failures... thus:

if random = 666 then
[dothisandthat]
endif

To make it rarer, you can add another variable and make it
count up to a given threshold.

I updated the doc with that and am also working on a
description for the native phidget support as well (beyond
the phidgets.txt file comments).

Ciao

Enrico