sd-rtnl: fix reading of nla type

We must filter out the 'network-byteorder' and 'nested' flags.
sd-rtnl: mark nested attributes with the NLA_F_NESTED flag

The kernel mostly does not check this, but let's be consisntent and allways set it anyway. Based
on patch from Susant Sahani.
sd-rtnl: add support to set packet family type

This patch adds functionality to set family type
in the rtnl message for example PF_BRIDGE.
sd-rtnl: add bridge port rtnl attributes.

Add bridge port attributes to sd-rtnl to configure
via networkd.