This would be a structure holding all the data used to define a complete
network setup in Netplan. At this moment there are no guarantee of
coherence or validation of the validation, though.

We use some linker tricks to keep the previous globals `netdef`,
`netdef_ordered` and `global_ovs_config` visible in the ABI, even though
they now actually point to fields within a global state instance that
is there only for this compatibility purpose.

Note that the constructor, destructor and netdef accessors aren't
implemented yet. This is mostly because there are no consumers for this
API yet, and I'd like to get this commit into the mainline as is, which
means retaining 100% code coverage... ¯\_(ツ)_/¯

V2:
* rename the nps arguments to np_state
* use a netplan_state_* naming scheme for all functions pertaining to
  the new structure
* Add placeholder implementations for netplan_state_{new,clear}
* Add a comment in netplan_state_reset regarding the ownership of netdef
  objects
* Remove superfluous headers in abi_compat.c
* Remove the dependency on the error-code-flow branch

V3:
* Disable LCOV coverage on the placeholder functions