seccomp: Add tgid and tid into seccomp_data

Add the current thread and thread group IDs into the data
available for seccomp-bpf programs to work on.  This allows
installation of filters that police syscalls based on thread
or process ID, e.g. tgkill(2)/kill(2)/prctl(2).

Signed-off-by: David Drysdale <drysdale@google.com>