bgpd: fix hardset l3vpn label available in mpls pool

Today, when configuring BGP L3VPN mpls, the operator may
use that command to hardset a label value:

> router bgp 65500 vrf vrf1
> address-family ipv4 unicast
> label vpn export <hardset_label_value>

Today, BGP uses this value without checks, leading to potential
conflicts with other control planes like LDP. For instance, if
LDP initiates with a label chunk of [16;72] and BGP also uses the
50 label value, a conflict arises.

The 'label manager' service in zebra oversees label allocations.
While all the control plane daemons use it, BGP doesn't when a
hardset label is in place.

This update fixes this problem. Now, when a hardset label is set for
l3vpn export, a request is made to the label manager for approval,
ensuring no conflicts with other daemons. But, this means some existing
BGP configurations might become non-operational if they conflict with
labels already allocated to another daemon but not used.

note: Labels below 16 are reserved and won't be checked for consistency
by the label manager.

Fixes: ddb5b4880ba8 ("bgpd: vpn-vrf route leaking")
Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>

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