sanity: check for working user namespaces

If user namespaces are not available (typically because AppArmor is
blocking them), alert the user.

We consider network isolation sufficiently important that this is a fatal
error, and the user will need to configure AppArmor to allow bitbake to
create a user namespace.

[ YOCTO #15592 ]

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>