Core pid namespace enhancements

The following patchset updates the pid namespace infrastructure
so we don't constantly have to worry if we have been called
before or after exit_task_namespaces, by using the pid_namespace
obtained from a processes pid, handles the general case of setting
si_pid in struct sig_info, changes where we drop signals sent to init,
and enhances that changes to also work with the per namespace init.

Thus resolving most of the big gotchas with the current pid namespace
implementation.

Eric