virprocess: Introduce our own setns() wrapper

virprocess: Introduce our own setns() wrapper

From time to time weird bugreports occur on the list, e.g [1].
Even though the kernel supports setns syscall, there's an older
glibc in the system that misses a wrapper over the syscall.
Hence, after the configure phase we think there's no setns
support in the system, which is obviously wrong. On the other
hand, we can't rely on linux distributions to provide newer glibc
soon. Therefore we need to introduce the wrapper on or own.

1: https://www.redhat.com/archives/libvir-list/2014-September/msg00492.html

Signed-off-by: Stephan Sachse <ste.sachse@gmail.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>