d64 re-save file doesn't free original space

[davervw]

Background: d64 implementation supports overwriting, and apparently doesn't have a delete() method, because was a minimal implementation.

But it appears that re-saving a file will allocate more and more space on disk the more times it is saved, without freeing the original space. Workaround is to perform collection operation on the d64 with another tool. (validate via vice c1541 program)

Fix requires deleting first, or freeing storage after reinitializing directory entry while maintaining overwrite feature