Add a hashmap to the Manager struct that stores the association
between an IPv6 prefix and the network Link it is assigned to.
This is added in order to keep assigning the same prefixes with
the same links even though they are delegated at different times
or by different DHCPv6 clients.
Routing Policy rule manipulates rules in the routing policy database control the
route selection algorithm.
This work supports to configure Rule
```
[RoutingPolicyRule]
TypeOfService=0x08
Table=7
From= 192.168.100.18
```
```
ip rule show
0: from all lookup local
0: from 192.168.100.18 tos 0x08 lookup 7
```
V2 changes:
1. Added logic to handle duplicate rules.
2. If rules are changed or deleted and networkd restarted
then those are deleted when networkd restarts next time
V3:
1. Add parse_fwmark_fwmask
This will allow us to have several managers sharing an event loop
and running in parallel, as if they were running in separate processes.
The long term-aim is to allow networkd to be split into separate
processes, so restructure the code to make this simpler.
For now we drop the exit-on-idle logic, as this was anyway severely
restricted at the moment. Once split, we will revisit this as it may
then make more sense again.
If setting the received timezone or transient hostname fails because D-Bus is
not (yet) up, store the data in the Manager object and try again after
connecting to D-Bus.
