Every dynamic BGP was torn down on reconfig because the inherited
configuration is a little bit different than the parent one. Fixed this
by applying the same changes before the reconfiguration's memcmp().

This bug was introduced in 56af56a5.

Also fixed interface pattern reconfiguration which always restarted.
Added not only comparison but also actual reconfiguration of the pattern
itself so that one can update the pattern without restarting a running
BGP session.

Finally, extended documentation a bit to cover dynamic BGP scenarios a
little bit better. Yet, it probably deserves a separate section on
dynamic BGP.