bootctl: add new --print-root-device option

We already have this nice code in system that determines the block
device backing the root file system, but it's only used internally in
systemd-gpt-generator. Let's make this more accessible and expose it
directly in bootctl.

It doesn't fit immediately into the topic of bootctl, but I think it's
close enough and behaves very similar to the existing "bootctl
--print-boot-path" and "--print-esp-path" tools.

If --print-root-device (or -R) is specified once, will show the block device
backing the root fs, and if specified twice (probably easier: -RR) it
will show the whole block device that block device belongs to in case it
is a partition block device.

Suggested use:

        # cfdisk `bootctl -RR`

To get access to the partition table, behind the OS install, for
whatever it might be.

src/shared/blockdev-util.h

@@ -56,3 +56,5 @@ int block_device_has_partitions(sd_device *dev);
 int blockdev_reread_partition_table(sd_device *dev);
 
 int blockdev_get_sector_size(int fd, uint32_t *ret);
+
+int blockdev_get_root(int level, dev_t *ret);