native: Try hard to not malloc on a forked child

This appears to be causing the BSD bots to lock up when looking at the core
dumps I've managed to get. Dropping the `FileDesc` structure triggers the `Arc`
it's contained in to get cleaned up, invoking free(). This instead just closes
the file descriptor (the arc itself is never cleaned up).

I'm still not entirely sure why this is a problem because the pthreads runtime
should register hooks for fork() to prevent this sort of deadlock, but perhaps
that's only done on linux?