Check for possible bgpid collision on open. RFC 6286 defines that this

Check for possible bgpid collision on open. RFC 6286 defines that this
check needs to be done for iBGP sessions but not for eBGP sessions.
On conflict log an error and send a notification with ERR_OPEN_BGPID
suberror.
Reported by cjt (melissa_cjt at 163.com)
OK benno@