serve nfs: implement --nfs-cache-type symlink

`--nfs-cache-type symlink` is similar to `--nfs-cache-type disk` in
that it uses an on disk cache, but the cache entries are held as
symlinks. Rclone will use the handle of the underlying file as the NFS
handle which improves performance.

cmd/serve/nfs/symlink_cache_other.go

@@ -0,0 +1,8 @@
+//go:build unix && !linux
+
+package nfs
+
+// Turn the diskHandler into a symlink cache
+func (dh *diskHandler) makeSymlinkCache() error {
+	return ErrorSymlinkCacheNotSupported
+}