Added
* Change log that leaves minimal trace of changes updater performed
  stored in system.
* Support for `FilesSignature` field in packages. On mismatch it trigger
  reinstall.
* Support for `files-sha256sum` in packages as hash index

Fixed
* Subprocess call is now terminated way earlier thanks to `SIGCHLD`
  signal handling. This improves update time for any scripts spawning
  "daemon" processes that do not correctly redirect or close standard
  outputs.
* Reinstall of packages if some fields were formated differently in
  index than in package (updater no longer detects change by simple
  string comparison)

Changed
* Internal implementation of base64 replaced with base64c library.
* Utility opkg-create template and input format

Removed
* `--state-log` argument
* `--task-log` argument