sd-rtnl:introduce table-based lookup and typesafe read() functions

This patch introduces new netlink attribute parsing logic
which is table based lookup and sd_rtnl_message_read_*
methods for reading attributes. By doing this user does not
have to loop for the attribute values . Only providing the
attribute type it gets the attribute values which is optimized
and sd_rtnl_message_read_* methods are simplified.