Merge branch 'interface-for-routes'

Merge branch 'interface-for-routes'

Changes how the interface for routes installed with policies is
determined.  In most cases we now use the interface over which we reach the
other peer, not the interface on which the local address (or the source IP) is
installed.  However, that might be the same interface depending on the
configuration (i.e. in practice there will often not be a change).

Routes are not installed anymore for drop policies and for policies with
protocol/port selectors.

Fixes #809, #824, #1347.