Fix Postfix localhost destination after hostname domain change

- When the system hostname domain changes, update `localhost.<old-domain>` in Postfix `mydestination` to `localhost.<new-domain>`.

- This sits alongside the existing hostname/FQDN updates for Postfix destinations.

Previous behavior:

`save_dns.cgi` only updated Postfix `mydestination` entries that exactly matched:

- the old short hostname, like `host`
- the old FQDN, like `host.old-domain.test`

It did **not** update:

- `localhost.old-domain.test`

So if you changed:

```text
host.old-domain.test
```

to:

```text
host.new-domain.test
```

Postfix could become:

```text
mydestination = host.new-domain.test, host, localhost.old-domain.test
```

After this hunk, it also updates that localhost domain entry:

```text
localhost.old-domain.test
```

to:

```text
localhost.new-domain.test
```

net/save_dns.cgi

@@ -124,6 +124,19 @@ if (&foreign_installed("postfix") && $in{'hostname'} ne $old_hostname) {
 		&postfix::set_current_value("mydestination",
 					    join(", ", @mydests));
 		}
+	$old_domain = $old_hostname =~ /^[^\.]+\.(.*)$/ ? $1 :
+		      $old_fqdn =~ /^[^\.]+\.(.*)$/ ? $1 : undef;
+	$new_domain = $in{'hostname'} =~ /^[^\.]+\.(.*)$/ ? $1 :
+		      $new_fqdn =~ /^[^\.]+\.(.*)$/ ? $1 : undef;
+	if ($old_domain && $new_domain &&
+	    lc($old_domain) ne lc($new_domain)) {
+		$idx = &indexoflc("localhost.$old_domain", @mydests);
+		if ($idx >= 0) {
+			$mydests[$idx] = "localhost.$new_domain";
+			&postfix::set_current_value("mydestination",
+						    join(", ", @mydests));
+			}
+		}
 
 	# Update postfix myorigin
 	$myorigin = &postfix::get_current_value("myorigin");