Revert "freezer: rename thaw_process() to __thaw_task() and simplify the implementation thaw_process() now has only internal users - system and cgroup freezers. Remove the unnecessary return value, rename, unexport and collapse __thaw_process() into it. This will help further updates to the freezer code."

This reverts commit ff0318aa2d268d715ac3b55977813912380d7660.