NM_UTILS_LOOKUP_STR() uses alloca(). Partly to avoid the overhead of
malloc(), but more important because it's convenient to use. It does
not require to declare a varible to manage the lifetime of the heap
allocation.

It's quite safe, because the stack allocation is of a fixed size of only
a few bytes. Overall, I think the convenience that we get (resulting in
simpler code) outweighs the danger of stack allocation in this case. It's
still worth it.
However, as it uses alloca(), it still must not be used inside a (unbound)
loop and it is obviously a macro.

Rename the macros to have a _A() suffix. This should make the
peculiarities more apparent.