Also fix numberous bugs, such as:
- Not getting confused on more flags taking file arguments.
- Ensuring children reexport their children, but the original
binary/library doesn't.
- Not spawning children when it turns out we just dynamically link
under the threshold but our total number of inputs exceeeds it.
- Children were always named `libunnamed-*`, when that name was
supposed to be the last resort only.
ld-wrapper's own RPATH check hardcodes `.so`, but darwin uses `.dylib`
*and* (in practice due to lousy build systems) `.so`. We don't care
however because we never inject `--rpath` like that in practice on
Darwin. Hopefully someday we won't on linux either.
1a72330ab0