rtnl: add preliminary support for containers

For now, we only support one container type IFLA_LINKINFO, and we
still lack support for parsing the containers again.
rtnl: add support for IFLA_MASTER
networkd: minor fixes

In particular, store the ifname, though we should only use it carefully, as
it is not guaranteed to be stable. Using it for logging is fine though.
networkd: add bridge support

A bridge is specified in a .netdev file with a section [Bridge]
and at least the entry Name=.

A link may be joined to a bridge if the .network applied to it has
a Bridge= entry giving the name of the bridge in its [Network] section.

We eagerly create all bridges on startup, and links are added to
bridges as soon as they both appear.