fix encrypt-on-receive with placeholders

fixes https://github.com/zrepl/zrepl/issues/504

Problem:
  plain send + recv with root_fs encrypted + placeholders causes plain recvs
  whereas user would expect encrypt-on-recv
Reason:
  We create placeholder filesytems with -o encryption=off.
  Thus, children received below those placeholders won't inherit
  encryption of root_fs.
Fix:
  We'll have three values for `recv.placeholders.encryption: unspecified (default) | off | inherit`.
  When we create a placeholder, we will fail the operation if  `recv.placeholders.encryption = unspecified`.
  The exception is if the placeholder filesystem is to encode the client identity ($root_fs/$client_identity) in a pull job.
  Those are created in `inherit` mode if the config field is `unspecified` so that users who don't need
  placeholders are not bothered by these details.

Future Work:
  Automatically warn existing users of encrypt-on-recv about the problem
  if they are affected.
  The problem that I hit during implementation of this is that the
  `encryption` prop's `source` doesn't quite behave like other props:
  `source` is `default` for `encryption=off` and `-` when `encryption=on`.
  Hence, we can't use `source` to distinguish the following 2x2 cases:
  (1) placeholder created with explicit -o encryption=off
  (2) placeholder created without specifying -o encryption
  with
  (A) an encrypted parent at creation time
  (B) an unencrypted parent at creation time

daemon/job/build_jobs_sendrecvoptions.go

@@ -72,6 +72,16 @@ func buildReceiverConfig(in ReceivingJobConfig, jobID endpoint.JobID) (rc endpoi
 		return rc, errors.Wrap(err, "cannot build bandwith limit config")
 	}
 
+	placeholderEncryption, err := endpoint.PlaceholderCreationEncryptionPropertyString(recvOpts.Placeholder.Encryption)
+	if err != nil {
+		options := []string{}
+		for _, v := range endpoint.PlaceholderCreationEncryptionPropertyValues() {
+			options = append(options, endpoint.PlaceholderCreationEncryptionProperty(v).String())
+		}
+		return rc, errors.Errorf("placeholder encryption value %q is invalid, must be one of %s",
+			recvOpts.Placeholder.Encryption, options)
+	}
+
 	rc = endpoint.ReceiverConfig{
 		JobID:                      jobID,
 		RootWithoutClientComponent: rootFs,
@@ -81,6 +91,8 @@ func buildReceiverConfig(in ReceivingJobConfig, jobID endpoint.JobID) (rc endpoi
 		OverrideProperties: recvOpts.Properties.Override,
 
 		BandwidthLimit: bwlim,
+
+		PlaceholderEncryption: placeholderEncryption,
 	}
 	if err := rc.Validate(); err != nil {
 		return rc, errors.Wrap(err, "cannot build receiver config")