Remove PIPEXCSESSION pipex(4) ioctl(2) command from kernel and man page.

Remove PIPEXCSESSION pipex(4) ioctl(2) command from kernel and man page.

Long time ago pipex(4) session can't be deleted until both pipex(4)
input and output queues become empty. Dead sessions were linked to the
stack and the `ip_forward' flag was used to prevent packets forwarding.
npppd(8) marked such sessions by doing PIPEXCSESSION ioctl(2) call.

But since we started to unlink close session from the stack, this logic
became unnecessary. Also pipex(4) session could be closed just after
close request.

npppd(8) was the only userland program which did PIPEXCSESSION ioctl(2)
call, and we removed it week ago. It's time to remove the remains.

Now the `flags' member of 'pipex_session' structure became immutable.

ok yasuoka@