session: Use strings as bearer type

Instead of using 'enums' for the allowed bearer we drop back
and use strings again. The main reason is that we want also
to support service name as valid 'bearer type' in the policy plugin.
The D-Bus API will still only accept real bearer types.
session: Add service name matching for policy plugin

The policy plugin is able to add service idents to the AllowedBearers
list and now we also allow to match on it.
session: Add FilterBearers

FilterBearers allows the user to remove a service which would be
incluced via AlloweBearers, e.g.

AllowedBearers = [ ethernet ]
FilterBearers = [ ethernet_123456_cable ]

So all services of type ethernet are acceptable except the
ethernet_123456_cable one.