makechrootpkg: sync_chroot: Make more general.

This is inspired by the thought that went in to the delete_chroot
is_subvolume commit.

sync_chroot($chrootdir, $copydir) copies `$chrootdir/root` to `$copydir`.
That seems a little silly; why do we care about "$chrootdir"?  Have it just
be sync_chroot(source, destination) like every other sync/copy command.

Where this becomes tricky is check to decide if we are going to use btrfs
subvolumes or not.  We don't care if "$source/.." is on btrfs; the root
could be a directly-mounted subvolume, but and the destination could be
another subvolume of the same btrfs mounted somewhere else.

The things we do care about are:

 - The source is a btrfs subvolume (so that we can snapshot it)
 - The source is on the same filesystem as the directory that the copy will
   be created in.
 - If the destination exists:
   * that it is not a mountpoint (so that we can delete and recreate it)
   * that it is a btrfs subvolume (so that we can quickly delete it)

On the last point, it isn't necessary for creating the new snapshot, just
for quick deletion.  That can be a separate check, where we use regular
`rm` for deleting the existing copy, but use subvolume snapshots for
creating the new one.

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