networkd: manager - refactor link tracking a bit
networkd: netdev - rephrase logging message a bit

Make it fit with what is logged from the link.
networkd: manager - initialize variables
networkd: network - fix leak
sd-rtnl: message - add support for getting prefixlen from address messages
networkd: link - redo flag change logging

Make the logging less verbose by only printing all the changed flags on one line,
at the same time make it more complete by supporting all flags currently supported
by the kernel.

We still fall back to printing the raw flags in case we get something we do not recognize
This may be useful when running on new kernels.
rtnl: change from bitmask to enum for rtnl groups

The bitmask is deprecated in the kernel, so move to the new interface. At the moment
this does not make a difference for us, but it avoids having to change the API in the future.
rtnl: message - move code around

No functional change.
rtnl: message - verify that we read the pending message size from the kernel

Reuse the auth-checking for both the peek and the real read.
rtnl: message - read group membership of incoming messages
rtnl: message - fix check for broadcast messages
networkd: listen for address changes

This is proof-of-concept only, as we only log the changes but don't do anything
with it.