MacOS: rely on OSXFuse to create mountpoint below /Volumes

OSXFuse automatically creates the mountpoint if it is
below /Volumes because this would require root permissions
which the user might not have.

Reported at https://github.com/rfjakob/gocryptfs/issues/194

checkdir.go

@@ -6,7 +6,8 @@ import (
 	"os"
 )
 
-// checkDirEmpty - check if "dir" exists and is an empty directory
+// checkDirEmpty - check if "dir" exists and is an empty directory.
+// Returns an *os.PathError if Stat() on the path fails.
 func checkDirEmpty(dir string) error {
 	err := checkDir(dir)
 	if err != nil {

mount.go

@@ -58,6 +58,12 @@ func doMount(args *argContainer) int {
 		err = checkDir(args.mountpoint)
 	} else {
 		err = checkDirEmpty(args.mountpoint)
+		// OSXFuse will create the mountpoint for us if it is below /Volumes/
+		if runtime.GOOS == "darwin" && os.IsNotExist(err) && strings.HasPrefix(args.mountpoint, "/Volumes/") {
+			tlog.Info.Printf("Mountpoint %q does not exist, but should be created by OSXFuse",
+				args.mountpoint)
+			err = nil
+		}
 	}
 	if err != nil {
 		tlog.Fatal.Printf("Invalid mountpoint: %v", err)