Adjust lock requirements for ps_pptr, ps_ppid and ps_oppid.

Adjust lock requirements for ps_pptr, ps_ppid and ps_oppid.

ps_pptr, ps_ppid and ps_oppid require the KERNEL_LOCK and the process mutex
to be modified. At the same time either KERNEL_LOCK or process mutex needs
to be taken to read the values.

This is needed to further unlock ptsignal().
OK kettenis@, mvs@