Adapted, to the c64 target, the INIT-segment overlay scheme from the apple2 targets.

Adapted, to the c64 target, the INIT-segment overlay scheme from the apple2 targets.

When a program starts running, INIT is moved from one place to another place.  Then, INIT's code is executed; and, the first place is re-used for variables.  After the INIT code has finished, the second place can be re-used by the heap and the C stack.  That means that initiation code and data won't waste any RAM space after they stop being needed.