Ports the added system calls from NIOFS (from #3505) to _NIOFileSystem:

Adds three new system call wrappers for the symlinkat, renameatx_np, and unlinkat system calls.

Motivation:

_NIOFileSystem module is around for backward compatibility - hence, we are porting the new feature to it.

Modifications:

Exactly same as in #3505 but applied to the _NIOFileSystem:

Related to #3403 and #3470. This PR adds syscall wrappers needed to atomically overwrite existing files or symlinks at the destination during copy operations. On Linux, atomic overwrites require a "copy to temp file, then rename" strategy. We use the *at family of syscalls (which operate relative to directory file descriptors) to avoid TOCTOU race conditions.