sd-rtnl: add sd_rtnl_message_enter_container()

Extend rta_offset_tb into a stack of offset tables, one for each parent of the
current container, and make sd_rtnl_message_{enter,exit}_container() pop/push
to this stack.

Also make sd_rtnl_message_rewind() parse the top-level container, and use this
when reading a message from the socket.

This changes the API by dropping the now redundant sd_rtnl_message_read()
method.
networkd: netdev - verify that newlink messages has the expected kind

We match 'newlink' messages with expected netdev's based on their names. Now also
make sure that the receieved link has the expected kind.
sd-rtnl: message parsing - log when ignoring message attributes