unshare,nsenter: Move the old libc handling into a common header namespace.h

unshare,nsenter: Move the old libc handling into a common header namespace.h

Move the defitions of CLONE_NEWNS, CLONE_NEWUTS, CLONE_NEWIPC,
CLONE_NEWNET, CLONE_NEWUSER, CLONE_NEWPID into namespace.h in case
sched.h does not provide those definitions.  Are there systems
around that are old enough that still need this?

Move the definitions of unshare() and setns() into namespace.h
for supporting old versions of libc that does not provice these.
I have tested this support with setns as I still have systems
old enough that glibc does not wrap setns.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>