What works:

  nmcli c add con-name dum0 ifname dum0 type dummy \
      tc.qdiscs 'ingress, root pfifo_fast'
  nmcli c modify dum0 -tc.qdiscs 'root pfifo_fast'
  nmcli c modify dum0 +tc.qdiscs 'root handle 666: fq_codel'

This only adds new API for a NMSettingUser. The setting class
is still entirely unused.

The point is getting the new API into 1.8.0 release of libnm.
It's easier to backport the use of the API to a stable branch
then backporting public API.

https://bugzilla.gnome.org/show_bug.cgi?id=776276
https://bugzilla.redhat.com/show_bug.cgi?id=1421429

In practice, this should only matter when there are multiple
header files with the same name. That is something we try
to avoid already, by giving headers a distinct name.

When building NetworkManager itself, we clearly want to use
double-quotes for including our own headers.
But we also want to do that in our public headers. For example:

  ./a.c
    #include <stdio.h>
    #include <nm-1.h>
    void main() {
        printf ("INCLUDED %s/nm-2.h\n", SYMB);
    }

  ./1/nm-1.h
    #include <nm-2.h>

  ./1/nm-2.h
    #define SYMB "1"

  ./2/nm-2.h
    #define SYMB "2"

$ cc -I./2 -I./1 ./a.c
$ ./a.out
INCLUDED 2/nm-2.h

Exceptions to this are
  - headers in "shared/nm-utils" that include <NetworkManager.h>. These
    headers are copied into projects and hence used like headers owned by
    those projects.
  - examples/C