Handle `AT_FDCWD` in C to eliminate `Obj.magic`

[dra27]

Eliminates both the need to probe AT_FDCWD and the need to use Obj.magic in the library to pass it to the C stubs.

If my understanding of the calling convention for optional arguments is correct, even with the default in the code and a non-opaque interface, the option is still allocated at call-sites, so I don't think the allocation profile changes.