Releases all heap blocks in the free heap list to the operating system.
DesignVision allocates memory in blocks at least as big as specified in
dv.ini/trim.ini with the heap_block_size keyword
(default: 4000). When a memory block is no longer needed, for example when a
list is freed, the memory blocks are placed in a free heap list. New allocations
search this list first. This eliminates any potential memory fragmentation
problems. If an application uses a large amount of memory, for example by
many very large lists and then freeing them, on a system with many users, then
it might be beneficial to return this unused memory to the operating system
by calling ret_freeheap(). ret_freeheap() also returns
of freed heap blocks.