start, utils: improve preserve_ns()

start, utils: improve preserve_ns()

- Allocating an error message that the caller must free seems pointless. We can
  just print the error message in preserve_ns() itself. This also allows us to
  avoid using the GNU extension asprintf().
- Improve lxc_preserve_ns(): By passing in NULL or "" as the second argument
  the function can now also be used to check whether namespaces are supported
  by the kernel.
- Use lxc_preserve_ns() in preserve_ns().

Signed-off-by: Christian Brauner <christian.brauner@canonical.com>