handle changes to placeholder state correctly

We assumed that `zfs recv -F FS` would basically replace FS inplace, leaving its children untouched.
That is in fact not the case, it only works if `zfs send -R` is set, which we don't do.

Thus, implement the required functionality manually.

This solves a `zfs recv` error that would occur when a filesystem previously created as placeholder on the receiving side becomes a non-placeholder filesystem (likely due to config change on the sending side):

  zfs send pool1/foo@1 | zfs recv -F pool1/bar
  cannot receive new filesystem stream:
  destination has snapshots (eg. pool1/bar)
  must destroy them to overwrite it

replication/logic/pdu/pdu.proto

@@ -14,12 +14,12 @@ message ListFilesystemReq {}
 
 message ListFilesystemRes {
     repeated Filesystem Filesystems = 1;
-    bool Empty = 2;
 }
 
 message Filesystem {
     string Path = 1;
     string ResumeToken = 2;
+    bool IsPlaceholder = 3;
 }
 
 message ListFilesystemVersionsReq {