CGROUP_SOCK_ADDR support?

First of all, thanks a lot for `bcc`. It its an awesome tool to easily experiment with `eBPF`!

I recently needed to load an `CGROUP_SOCK_ADDR` program from Python. It turns out that it not currently possible as the `expected_attach_type` member of the `bpf_load_program_attr` structure must be filled before calling `bcc_prog_load_xattr()`. I successfully modified `BPFModule::bcc_func_load()` and I am no able to load `CGROUP_SOCK_ADDR` programs.

I am willing to provide a patch, but I need advice to do it. Shall I modify `BPFModule::bcc_func_load()` to accept an `expected_attach_type` argument (i.e. that will break the current API), or shall I add `BPFModule::bcc_func_load_xattr()` that will get a pointer to a `bpf_load_program_attr` structure (i.e. that will be more generic and future proof)?