Pull request: home: mv local domain name to dhcp setts

Closes #3367.

Squashed commit of the following:

commit e2cabcac2d91af24b9e5f4ac8f78daf5e8d339b9
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Jan 25 19:35:31 2022 +0300

    home: imp test, skip dhcp test on windows

commit e58053f11e081630ad4e8d1e77a7a74226029db0
Merge: ff2fe87d 0b72bcc5
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Jan 25 19:10:25 2022 +0300

    Merge branch 'master' into 3367-dhcp-local-domain-name

commit ff2fe87d8cab12e60d045be636e366e392d6d96f
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Dec 30 20:55:34 2021 +0300

    home: mv local domain name to dhcp setts

internal/home/upgrade.go

@@ -21,7 +21,7 @@ import (
 )
 
 // currentSchemaVersion is the current schema version.
-const currentSchemaVersion = 12
+const currentSchemaVersion = 13
 
 // These aliases are provided for convenience.
 type (
@@ -85,6 +85,7 @@ func upgradeConfigSchema(oldVersion int, diskConf yobj) (err error) {
 		upgradeSchema9to10,
 		upgradeSchema10to11,
 		upgradeSchema11to12,
+		upgradeSchema12to13,
 	}
 
 	n := 0
@@ -690,6 +691,52 @@ func upgradeSchema11to12(diskConf yobj) (err error) {
 	return nil
 }
 
+// upgradeSchema12to13 performs the following changes:
+//
+//   # BEFORE:
+//   'dns':
+//     # …
+//     'local_domain_name': 'lan'
+//
+//   # AFTER:
+//   'dhcp':
+//     # …
+//     'local_domain_name': 'lan'
+//
+func upgradeSchema12to13(diskConf yobj) (err error) {
+	log.Printf("Upgrade yaml: 12 to 13")
+	diskConf["schema_version"] = 13
+
+	dnsVal, ok := diskConf["dns"]
+	if !ok {
+		return nil
+	}
+
+	var dns yobj
+	dns, ok = dnsVal.(yobj)
+	if !ok {
+		return fmt.Errorf("unexpected type of dns: %T", dnsVal)
+	}
+
+	dhcpVal, ok := diskConf["dhcp"]
+	if !ok {
+		return nil
+	}
+
+	var dhcp yobj
+	dhcp, ok = dhcpVal.(yobj)
+	if !ok {
+		return fmt.Errorf("unexpected type of dhcp: %T", dnsVal)
+	}
+
+	const field = "local_domain_name"
+
+	dhcp[field] = dns[field]
+	delete(dns, field)
+
+	return nil
+}
+
 // TODO(a.garipov): Replace with log.Output when we port it to our logging
 // package.
 func funcName() string {