sysd-rules generation doesn't allow non-invasive addition of addon rules

[Roland McGrath]

Please try what I just committed.  nptl/Makeconfig has the example of what
you have to define to add your flavor.  Those bits can probably go into a
sysdeps Makefile instead of the add-on having a Makeconfig, shouldn't matter.

The changes I've made affect the existing order of rules generated sysd-rules,
but should not make any practical difference.  Folks please keep an eye out
for if I broke something.


Thanks,
Roland