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

zfs/placeholder.go

@@ -105,3 +105,9 @@ func ZFSCreatePlaceholderFilesystem(p *DatasetPath) (err error) {
 
 	return
 }
+
+func ZFSSetNoPlaceholder(p *DatasetPath) error {
+	props := NewZFSProperties()
+	props.Set(ZREPL_PLACEHOLDER_PROPERTY_NAME, "off")
+	return zfsSet(p.ToString(), props)
+}