More robust memory limits

At present I am setting memory limits manually.  I review the cc65 `.map` file and find the upper limit of the memory used by cc65 and then set my HEAP1 and HEAP2 limits accordingly.  Oliver Schmidt suggested a better way using `malloc()` and `_heapmaxavail`.  See cc65 issue #655 for more details.  This applies to Apple II only, as I am not linking in `malloc()` on VIC20/C64.