exit.c: unexport __set_special_pids()

Move __set_special_pids() from exit.c to sys.c close to its single caller
and make it static.

And rename it to set_special_pids(), another helper with this name has
gone away.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>