switch-root: don't require /mnt/ when switching root into host OS

So far, we invoked pivot_root() specifying /mnt/ as second argument,
which then unmounted right-after. We'd create /mnt/ if needed. This
sucks, because it means /mnt/ must strictly be pre-created on immutable
images.

Remove this limitation, by using pivot_root() with "." as source and
target, which will result in two stacked mounts afterwards: the new one
underneath, the old one ontop. We can then simply unmount the top one,
and have what we want without needing any extra /mnt/ dir.

Since we don't need /mnt/ anymore we can get rid of the extra
unmount_old_root parameter and simply specify it as NULL if we don't
want the old mount to stick around.

src/shared/switch-root.h

@@ -3,4 +3,4 @@
 
 #include <stdbool.h>
 
-int switch_root(const char *new_root, const char *oldroot, bool detach_oldroot, unsigned long mountflags);
+int switch_root(const char *new_root, const char *old_root_after, unsigned long mount_flags);