Arguably, it is more convenient to use the static buffer as
it saves typing.

But having such a low-level function use a static buffer also
limits the way how to use it. As it was, you could not avoid
using the static buffer.

E.g. you cannot do:

  char buf[100];

  _LOGD ("nmp-object: %s; platform-link: %s",
         nmp_object_to_string (nmpobj, buf, sizeof(buf)),
         nm_platform_link_to_string (link));

This will fail for non-obvious reasons because both
to-string functions end up using the same static buffer.

Also change the to-string implementations to accept NULL
as valid and return it as "(null)".

https://bugzilla.gnome.org/show_bug.cgi?id=756427