Append the childs pid to the name of the cgroup

Whereas before the cgroup would be created as e.g

    get_iplayer/

it is now created as eg

    get_iplayer-16663/

This allows multiple instances of the same program to be run each in
their own cgroup.

Signed-off-by: Andrew Clayton <andrew@digital-domain.net>
Only call getpid(2) once in the child

Get the pid of the child process once and use it rather than multiple
calls to getpid().

Signed-off-by: Andrew Clayton <andrew@digital-domain.net>