Clean up misc. todos.

- Implement missing extra field functions
- add flag for zip_file_add that overwrites any existing entry with same name
- set "version needed"/"version made by" correctly (zip64)
- zip_replace: set compression method to default unless user-set
- reset compression specific bits in bitflags when (re)compressing
- rename zip_get_{compression,encryption}_implementation to _zip_*
13 files changed