setns(): reimplement via syscall() if not available

1 Modify configure.ac to not fail if setns() is not available,
  and define HAVE_SETNS if it is

2 Reimplement setns syscall using syscall(). Defines for __NR_setns
  are taken from kernel headers.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
2 files changed