Remove variadic marcos for C89 conformance.

C99 introduced variadic macros.  I was using variadic macros to wrap fprintf in
NXN_DEBUG_LOG and NXN_EXCEPTION_ABORT.

The fix is to double-wrap the arguments:

    NXN_DEBUG_LOG("name:%s,age:%d\n", "Joe", 19); ->
        NXN_DEBUG_LOG((NXN_DEBUG_HDR "name:%s, age:%d\n", "Joe", 19));

and likewise for NXN_EXCEPTION_ABORT (uses NXN_EXCEPTION_HDR).

The logged statements no longer include the function name, since the __func__
macro is part of C99.

The resultant macros aren't as useful, but at least they're portable.  In any
case, neither of these macros should be used that often (since one is there to
help when debugging/developing, and the other is only for exceptional cases).