zebra: increase maximum label stack depth

zebra: increase maximum label stack depth

* Bump MPLS_MAX_LABELS from 2 to 16;
* Adjust the static_nh_label structure and the mpls_label2str() function;
* On OpenBSD, print an error message when trying to push more than one
  label at once (kernel limitation). While here, add support for MPLSv6
  FTNs in OpenBSD.

This is not the full package. We still can't pop multiple labels at once,
or do things like swap a label and push other ones. We'll address that
in the future.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>