Add support for cloning cgroup filter object.

In this commit, we implement ematch_tree_clone(), which is basis of
cgroup_clone() interface. The whole ematch tree is deep-copied except
the e_ops filed.

Also, a new unit test is added for testing the interface, which named as
check-ematch-tree-clone.c located in tests directory.

https://github.com/thom311/libnl/pull/176
8 files changed