-/* Concatenate two pathname components, DIR and BASE, in newly-allocated
- storage and return the result. Return 0 if out of memory. Add a slash
- between DIR and BASE in the result if neither would contribute one.
- If each would contribute at least one, elide one from the end of DIR.
- Otherwise, simply concatenate DIR and BASE. In any case, if
- BASE_IN_RESULT is non-NULL, set *BASE_IN_RESULT to point to the copy of
- BASE in the returned concatenation. */