sd-netlink: respect attribute type flags

Though currently unused by us, netlink attribute types support embedding flags to indicate
if the type is encoded in network byte-order and if it is a nested attribute. Read out
these flags when parsing the message.

We will now swap the byteorder in case it is non-native when reading out integers (though
this is not needed by any of the types we currently support). We do not enforce the NESTED
flag, as the kernel gets this wrong in many cases.