bgpd: Install SAFI_LABELED_UNICAST routes in SAFI_UNICAST table

bgpd: Install SAFI_LABELED_UNICAST routes in SAFI_UNICAST table

Signed-off-by: Daniel Walton <dwalton@cumulusnetworks.com>
- All ipv4 labeled-unicast routes are now installed in the ipv4 unicast
  table. This allows us to do things like take routes from an ipv4
  unicast peer, allocate a label for them and TX them to a ipv4
  labeled-unicast peer. We can do the opposite where we take routes from
  a labeled-unicast peer, remove the label and advertise them to an ipv4
  unicast peer.

- Multipath over a labeled route and non-labeled route is not allowed.

- You cannot activate a peer for both 'ipv4 unicast' and 'ipv4
  labeled-unicast'

- The 'tag' variable was overloaded for zebra's route tag feature as
  well as the mpls label. I added a 'mpls_label_t mpls' variable to
  avoid this.  This is much cleaner but resulted in touching a lot of
  code.