api: cg_build_path_locked() - support default systemd delegate scope

api: cg_build_path_locked() - support default systemd delegate scope

Add support to build cgroup path to the default systemd slice/scope
cgroup, if setdefault is set in the systemd configuration of the
cgconfig.conf.

$ cat /etc/cgconfig.conf
...
systemd {
    slice = database.slice;
    scope = db.scope;
    setdefault = yes; <--  set cgroup root path to '/sys/fs/cgroup/database.slice/db.scope/'
}

systemd {
    slice = database.slice;
    scope = house_keeping.scope;
    pid = 3241;
}

systemd {
    slice = others.slice;
    scope = server.scope;
}

cg_build_path() constructs the cgroup path to the mount point of the
given cgroup controller. With the support for the default systemd scope,
the default cgroup path for the controller, appends the slice/scope
name, making it the default cgroup root.

Allowing the users to set their default cgroup path from
'/sys/fs/cgroup/' to
'/sys/fs/cgroup/<systemd slice name>/<systemd scope name>/', while
using the libcgroup tools, allowing them to work on the delegated
cgroup hierarchy.

Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>