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

zfs/filesystemplaceholdercreateencryptionvalue_enumer.go

@@ -0,0 +1,51 @@
+// Code generated by "enumer -type=FilesystemPlaceholderCreateEncryptionValue -trimprefix=FilesystemPlaceholderCreateEncryption"; DO NOT EDIT.
+
+//
+package zfs
+
+import (
+	"fmt"
+)
+
+const _FilesystemPlaceholderCreateEncryptionValueName = "InheritOff"
+
+var _FilesystemPlaceholderCreateEncryptionValueIndex = [...]uint8{0, 7, 10}
+
+func (i FilesystemPlaceholderCreateEncryptionValue) String() string {
+	i -= 1
+	if i < 0 || i >= FilesystemPlaceholderCreateEncryptionValue(len(_FilesystemPlaceholderCreateEncryptionValueIndex)-1) {
+		return fmt.Sprintf("FilesystemPlaceholderCreateEncryptionValue(%d)", i+1)
+	}
+	return _FilesystemPlaceholderCreateEncryptionValueName[_FilesystemPlaceholderCreateEncryptionValueIndex[i]:_FilesystemPlaceholderCreateEncryptionValueIndex[i+1]]
+}
+
+var _FilesystemPlaceholderCreateEncryptionValueValues = []FilesystemPlaceholderCreateEncryptionValue{1, 2}
+
+var _FilesystemPlaceholderCreateEncryptionValueNameToValueMap = map[string]FilesystemPlaceholderCreateEncryptionValue{
+	_FilesystemPlaceholderCreateEncryptionValueName[0:7]:  1,
+	_FilesystemPlaceholderCreateEncryptionValueName[7:10]: 2,
+}
+
+// FilesystemPlaceholderCreateEncryptionValueString retrieves an enum value from the enum constants string name.
+// Throws an error if the param is not part of the enum.
+func FilesystemPlaceholderCreateEncryptionValueString(s string) (FilesystemPlaceholderCreateEncryptionValue, error) {
+	if val, ok := _FilesystemPlaceholderCreateEncryptionValueNameToValueMap[s]; ok {
+		return val, nil
+	}
+	return 0, fmt.Errorf("%s does not belong to FilesystemPlaceholderCreateEncryptionValue values", s)
+}
+
+// FilesystemPlaceholderCreateEncryptionValueValues returns all values of the enum
+func FilesystemPlaceholderCreateEncryptionValueValues() []FilesystemPlaceholderCreateEncryptionValue {
+	return _FilesystemPlaceholderCreateEncryptionValueValues
+}
+
+// IsAFilesystemPlaceholderCreateEncryptionValue returns "true" if the value is listed in the enum definition. "false" otherwise
+func (i FilesystemPlaceholderCreateEncryptionValue) IsAFilesystemPlaceholderCreateEncryptionValue() bool {
+	for _, v := range _FilesystemPlaceholderCreateEncryptionValueValues {
+		if i == v {
+			return true
+		}
+	}
+	return false
+}