Make the netlink code slightly more generic.

1. Support sending netlink dump requests for protocols that do
   not use a base data structure but only attributes.
2. Remove some constants from iproute.py since they are
   already in netlink.py.
3. Add a no-op implementation of MaybeDebugCommand that can be
   overridden by subclasses.
4. Remove the family argument from _ParseAttributes, since it was
   unused.
5. Add a flags argument to MaybeDebugCommand, since decoding xfrm
   commands requires knowing whether the command was a dump or
   not.

Bug: 34812052
Test: existing tests pass
Change-Id: I6919795b01090ef626965ac7643a109ea17742a8