Re-engineer TXT records for simplicity and better compliance (#1063)

TXT records are now handled different.

1. The raw input from dnsconfig.js is passed all the way to the provider. The provider can determine if it can or can't handle such records (auditrecords.go) and processes them internally as such.
2. The CanUseTXTMulti capability is no longer needed.

* DSPs now register a table of functions
* Use audits for txt record variations
* unit tests pass. integration fails.
* fix deepcopy problem
* rename to AuditRecordSupport
* Reduce use of TXTMulti
* Remove CanUseTXTMulti
* fix Test Skip
* fix DO
* fix vultr
* fix NDC
* msdns fixes
* Fix powerdns and cloudflare
* HEDNS: Fix usage of target field to resolve TXT handling (#1067)
* Fix HEXONET

Co-authored-by: Robert Blenkinsopp <robert@blenkinsopp.net>
Co-authored-by: Jakob Ackermann <das7pad@outlook.com>

providers/netcup/auditrecords.go

@@ -0,0 +1,13 @@
+package netcup
+
+import (
+	"github.com/StackExchange/dnscontrol/v3/models"
+	"github.com/StackExchange/dnscontrol/v3/pkg/recordaudit"
+)
+
+// AuditRecords returns an error if any records are not
+// supportable by this provider.
+func AuditRecords(records []*models.RecordConfig) error {
+	return recordaudit.TxtNotEmpty(records)
+	// Still needed as of 2021-03-01
+}

providers/netcup/netcupProvider.go

@@ -6,6 +6,7 @@ import (
 
 	"github.com/StackExchange/dnscontrol/v3/models"
 	"github.com/StackExchange/dnscontrol/v3/pkg/diff"
+	"github.com/StackExchange/dnscontrol/v3/pkg/txtutil"
 	"github.com/StackExchange/dnscontrol/v3/providers"
 )
 
@@ -16,12 +17,15 @@ var features = providers.DocumentationNotes{
 	providers.CanUsePTR:              providers.Cannot(),
 	providers.CanUseSRV:              providers.Can(),
 	providers.CanUseCAA:              providers.Can(),
-	providers.CanUseTXTMulti:         providers.Can(),
 	providers.CanGetZones:            providers.Cannot(),
 }
 
 func init() {
-	providers.RegisterDomainServiceProviderType("NETCUP", New, features)
+	fns := providers.DspFuncs{
+		Initializer:    New,
+		AuditRecordsor: AuditRecords,
+	}
+	providers.RegisterDomainServiceProviderType("NETCUP", fns, features)
 }
 
 // New creates a new API handle.
@@ -94,6 +98,8 @@ func (api *netcupProvider) GetDomainCorrections(dc *models.DomainConfig) ([]*mod
 
 	// Normalize
 	models.PostProcessRecords(existingRecords)
+	txtutil.SplitSingleLongTxt(dc.Records) // Autosplit long TXT records
+
 	differ := diff.New(dc)
 	_, create, del, modify, err := differ.IncrementalDiff(existingRecords)
 	if err != nil {