Showcase for the new macros NM_UTILS_STRING_LOOKUP_TABLE() and
NM_UTILS_STRING_LOOKUP_TABLE_DEFINE_STATIC().

It changes behavior in case of looking up an invalid/unknown
state reason. Previously it would just have returned "unknown"
-- which was indistinguishable from a regular "unknown" value.

Now it returns the numeric id as a string. The string is allocated
with alloca(), which is desired but one should be aware of the pitfalls:

- prevents the caller from being inlined
- bad idea to do in a loop.