Allow operators to enforce the presence of certain capabilities on sessions.

Allow operators to enforce the presence of certain capabilities on sessions.

For simple capabilities this just adds enforce to the yes/no option of the
announce statement. For multi-protocol capabilities and add-path there is
an extra keyword. On top of this for add-path the enforcement requires the
neighbor to send a matching capability, e.g
'announce add-path recv enforce' requires the other side to send any
'announce add-path send XYZ' capability.

This is mainly to enforce as-4byte and extra multi-protocol capabilities.

OK denis@ tb@