cgroup: rename cgroup_modify_nft_set() → unit_modify_nft_set()

cgroup: rename cgroup_modify_nft_set() → unit_modify_nft_set()

This is the only function that cgroup.h exports that is prefixed with
cgroup_ where this does not refer to some type such as CGroupContext or
CGroupTasksMax or so. It simply operates on a unit. And it doesn't even
modify a cgroup, but just modifies an nft set.

Hence, to make the naming scheme systematic, change prefix from cgroup_
to unit_, matching the majority of the functions that operate on Unit*
in the file.