Add a function which uses the unshare glibc call to disable networking
in the current process. This doesn't work on older distros/kernels
but will on more recent ones so for now we simply ignore the cases we
can't execute on. uid/gid can be passed in externally so this can
work with pseudo/fakeroot contexts.
(Bitbake rev: 9d6341df611a1725090444f6f8eb0244aed08213)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>