This more politely expresses the same thing

Source master rebase of my [PR NixOS#34].

Eventually, we might consider doing something for GNU binutils too, in
order that we switch (the normal) ld-wrapper to always use this to
leverage ld to resolve libraries, rather than faking it in bash.

[PR NixOS#34]: tpoechtrager/cctools-port#34

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.