Let's use the same common directory as the unit logic uses.

This means we have less to clean up, and opens the door to eventually
allow unprivileged operation of the
mount_image_privately_interactively() logic.

Addresses
https://github.com/systemd/systemd/pull/25608/commits/84be0c710d9d562f6d2cf986cc2a8ff4c98a138b#r1060130312,
https://github.com/systemd/systemd/pull/25608/commits/84be0c710d9d562f6d2cf986cc2a8ff4c98a138b#r1067927293, and
https://github.com/systemd/systemd/pull/25608/commits/84be0c710d9d562f6d2cf986cc2a8ff4c98a138b#r1067926416.

Follow-up for 84be0c710d9d562f6d2cf986cc2a8ff4c98a138b.

parse_boolean_argument() returns the same information via both the output
argument and normal return.

No functional change, just preparation for later commits.
These can be used in kernel-install later.

Note, unlike the our usual coding style, the arguments for
boot_entry_token_ensure() and parse_boot_entry_token_type() are
referenced, updated, and may freed, hence, always pass initialized
values. That's why they are not named as 'ret_xyz'.

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.