Zeropage Issues solved

Every Subroutine now has their own range of zp addresses assigned to
it. These are:

- scroll
- plot
- movebird
- screeninit

Comments show, which addresses are used within each subroutine (zp -
zp7). check source code :-)

Now it is no longer necessary to stop scrolling, when we reach the end
of our playfield :-)
Scrolling still buggy

It seam's. that the left border of the play field starts with x=4 and
not 0? Why?
Endless Scrolling works!
Sound is played, when bed dies……