nspawn: support ephemeral boots from images

Previously --ephemeral was only supported with container trees in btrfs
subvolumes (i.e. in combination with --directory=). This adds support for
--ephemeral in conjunction with disk images (i.e. --image=) too.

As side effect this fixes that --ephemeral was accepted but ignored when using
-M on a container that turned out to be an image.

Fixes: #4664

src/basic/missing.h

@@ -143,6 +143,10 @@
 #define GRND_RANDOM 0x0002
 #endif
 
+#ifndef FS_NOCOW_FL
+#define FS_NOCOW_FL 0x00800000
+#endif
+
 #ifndef BTRFS_IOCTL_MAGIC
 #define BTRFS_IOCTL_MAGIC 0x94
 #endif