bgpd: add cli command to control explicit-null label usage

bgpd: add cli command to control explicit-null label usage

In BGP labeled unicast address-family, it is not possible to
send explicit-null label values with redistributed or network
declared prefixes.
A new CLI command is introduced:

  > [no] bgp labeled-unicast explicit-null

When used, the explicit-null value for IPv4 ('0' value) or
IPv6 ('2' value) will be used.
It is necessary to reconfigure the networks or the
redistribution in order to inherit this new behaviour.

Add the documentation.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>