lib, zebra: support label chunk requests for SRGB

lib, zebra: support label chunk requests for SRGB

For SRGB, we need to support chunk requests starting at a
specific point in the label space, rather than just asking
for any sufficiently large chunk. To this purpose, we extend
the label manager api to request a chunk with a base value;
if the base is set to 0, the label manager will behave as it
currently does, i.e. fetching the first free chunk big enough
to satisfy the request.

update all the existing calls to get chunks from the label
manager so that they use MPLS_LABEL_BASE_ANY as the base
for the requested chunk

Signed-off-by: Emanuele Di Pascale <emanuele@voltanet.io>