We can do even better. We can remove the misguided code from
copy_process(CLONE_NEWPID) that populates the PIDTYPE_PGID/SID links
and generally does set setsid by hand, and the code from kernel_init
that call set_special_pid(), allowing us to remove the special case
The set_special_pid() in kernel_init() and the special case check
is actually a work around for the fact that earlier we could not
use 0 in the pid hash table. Now that we can use init_struct_pid
directly we don't need the special case at all.