bus-proxyd: add some more debugging

Make sure we print a message when exiting with an error.
networkd: improve logging a bit
networkd: don't automatically reload config at runtime

We don't know if the config will be consistent, so do as systemd itself and only
load config when the daemon starts (and possibly, in the future, when explicitly requested).
networkd: bridge - remove redundant state

We will not insist on getting the reply from rtnl that the bridge
was created before considering the bridge ready, as we will be
notified about that via udev. We will listen for the rtnl response
however, in case the creation of the bridge failed.
networkd: generate resolv.conf

This adds support to generate a basic resolv.conf in /run/systemd/network.
This file will not take any effect unless a symlink is created from
/etc/resolv.conf.

Nameservers received over DHCP takes precedence over statically configured ones.

Note: /etc/resolv.conf is severely limited, so in the future we will likely
rather provide a much more powerfull nss plugin (or something to that effect),
but this should allow current users to function without any loss of
functionality.
TODO: update rtnl/network sections
networkd: fix wording