Minor fixes to the migrator

Revert libcurl

libcurl has bug fixed by commit:
https://github.com/curl/curl/commit/a7b38c9dc98481e4a5fc37e51a8690337c674dfb
But new version is planned on February, so revert for now and wait for
fix.

Use libcurl and embed busybox

* libcurl is now used instead of curl command
* statically compiled busybox is embedded to updater library to be
  served instead of system one to use same busybox for whole updater run
* raise error when content is missing and allow its ignore
* fix collision when .list files also contained directories
* fix bug causing content miss to not be reported

Fixup: ignore directories in .list files

Fix re-execution not being done when replan specified

Fix inclusion of the variables introduced in v45.

Version 45 release

* Fix directories in .list files
* Add Updater version and additional variables to configuration