serve webdav: disallow on go1.8 due to dependent library changes

golang.org/x/net/webdav no longer builds with go1.8

cmd/serve/serve.go

@@ -13,7 +13,9 @@ import (
 
 func init() {
 	Command.AddCommand(http.Command)
-	Command.AddCommand(webdav.Command)
+	if webdav.Command != nil {
+		Command.AddCommand(webdav.Command)
+	}
 	Command.AddCommand(restic.Command)
 	if ftp.Command != nil {
 		Command.AddCommand(ftp.Command)

cmd/serve/webdav/webdav.go

@@ -1,3 +1,5 @@
+//+build go1.9
+
 package webdav
 
 import (

cmd/serve/webdav/webdav_test.go

@@ -3,7 +3,7 @@
 //
 // We skip tests on platforms with troublesome character mappings
 
-//+build !windows,!darwin
+//+build !windows,!darwin,go1.9
 
 package webdav
 

cmd/serve/webdav/webdav_unsupported.go

@@ -0,0 +1,11 @@
+// Build for webdav for unsupported platforms to stop go complaining
+// about "no buildable Go source files "
+
+// +build !go1.9
+
+package webdav
+
+import "github.com/spf13/cobra"
+
+// Command definition is nil to show not implemented
+var Command *cobra.Command = nil