cgroup: add foreign program to cgroup context

cgroup: add foreign program to cgroup context

- Store foreign bpf programs in cgroup context. A program is considered
foreign if it was loaded to a kernel by an entity external to systemd,
so systemd is responsible only for attach and detach paths.
- Support the case of pinned bpf programs: pinning to bpffs so a program
is kept loaded to the kernel even when program fd is closed by a user
application is a common way to extend program's lifetime.
- Aadd linked list node struct with attach type and bpffs path
fields.