bgpd: add [no]neighbor capability fqdn command

bgpd: add [no]neighbor capability fqdn command

cisco routers are not dealing fairly whith unsupported capabilities.
When a cisco router receive an unsupported capabilities it reset the
negociation without notifying the unmatching capability as described in
RFC2842.
Cisco suggest the use of
neighbor x.x.x.x capability fqdn
to avoid the use of fqdn in open message.

this new command is to remove the use of fqdn capability in the
open message with the peer "x.x.x.x".

Link: https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/116189-problemsolution-technology-00.pdf
Signed-off-by: Francois Dumontet <francois.dumontet@6wind.com>