- "gsystem-local-alloc.h" and <gio/gio.h> are already included via
  "nm-default.h". No need to include them separately.

- include "nm-macros-internal.h" via "nm-default.h" and drop all
  explict includes.

- in the modified files, ensure that we always include "config.h"
  and "nm-default.h" first. As second, include the header file
  for the current source file (if applicable). Then follow external
  includes and finally internal nm includes.

- include nm headers inside source code files with quotes

- internal header files don't need to include default headers.
  They can savely assume that "nm-default.h" is already included
  and with it glib, nm-glib.h, nm-macros-internal.h, etc.