We need to have lxc_attach() distinguish between a caller specifying specific
namespaces to attach to and a caller not requesting specific namespaces. The
latter is taken by lxc_attach() to mean that all namespaces will be attached.
This also needs to include all inherited namespaces.

Closes #1890.
Closes #1897.
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>