bgpd: bmp, add peer distinguisher support for peer up/down

bgpd: bmp, add peer distinguisher support for peer up/down

All BMP peer up/down messages send a 0:0 peer distinguisher.
This will not be ok when adding RD instance type.

Add code to get the peer distinguisher value.
- modify the API to pass the BGP instance instead of BMP.
- implement error cases with an unknown vrf identifier or a
peer type with local type value.
- handle the error return of the API; consequently, handle
the bmp_peerstate() error return in the calling functions.

There is no functional change, as the peer type value is
either loc-rib or global, both cases are already handled.

The next commit will handle the RD instance case.

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