sys_paccept: disable paccept() until API design is resolved

sys_paccept: disable paccept() until API design is resolved

The reasons for disabling paccept() are as follows:

* The API is more complex than needed.  There is AFAICS no demonstrated
  use case that the sigset argument of this syscall serves that couldn't
  equally be served by the use of pselect/ppoll/epoll_pwait + traditional
  accept().  Roland seems to concur with this opinion
  (http://thread.gmane.org/gmane.linux.kernel/723953/focus=732255).  I
  have (more than once) asked Ulrich to explain otherwise
  (http://thread.gmane.org/gmane.linux.kernel/723952/focus=731018), but he
  does not respond, so one is left to assume that he doesn't know of such
  a case.

* The use of a sigset argument is not consistent with other I/O APIs
  that can block on a single file descriptor (e.g., read(), recv(),
  connect()).

* The behavior of paccept() when interrupted by a signal is IMO strange:
  the kernel restarts the system call if SA_RESTART was set for the
  handler.  I think that it should not do this -- that it should behave
  consistently with paccept()/ppoll()/epoll_pwait(), which never restart,
  regardless of SA_RESTART.  The reasoning here is that the very purpose
  of paccept() is to wait for a connection or a signal, and that
  restarting in the latter case is probably never useful.  (Note: Roland
  disagrees on this point, believing that rather paccept() should be
  consistent with accept() in its behavior wrt EINTR
  (http://thread.gmane.org/gmane.linux.kernel/723953/focus=732255).)

I believe that instead, a simpler API, consistent with Ulrich's other
recent additions, is preferable:

accept4(int fd, struct sockaddr *sa, socklen_t *salen, ind flags);

(This simpler API was originally proposed by Ulrich:
http://thread.gmane.org/gmane.linux.network/92072)

If this simpler API is added, then if we later decide that the sigset
argument really is required, then a suitable bit in 'flags' could be added
to indicate the presence of the sigset argument.

At this point, I am hoping we either will get a counter-argument from
Ulrich about why we really do need paccept()'s sigset argument, or that he
will resubmit the original accept4() patch.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: David Miller <davem@davemloft.net>
Cc: Davide Libenzi <davidel@xmailserver.org>
Cc: Alan Cox <alan@redhat.com>
Cc: Ulrich Drepper <drepper@redhat.com>
Cc: Jakub Jelinek <jakub@redhat.com>
Cc: Roland McGrath <roland@redhat.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>