repart: Add BtrfsReplace= (#41109)

This is a series of commits which adds a feature needed by GNOME OS'
installer. This was show during All Systems Go 2025 talk:
https://cfp.all-systems-go.io/all-systems-go-2025/talk/QRJVL3/

To sum up this PR, this changes first systemd-repart to use BLKPG
partition instead of loop devices when possible. We need then to always
rescan the partitions to try remove partitions if it failed. We allow
encrypted partitions to stay activated and with a chosen name. And we
add a new partition configuration `BtrfsReplace=`.

Note that "replace" comes from the command `btrfs replace`. But in the
case of systemd-repart, maybe "inplace" or "move" would make more sense.
I open to suggestions.

If it is better I can split this into several PRs.

The commits:

## repart: Reuse the backing fd for fdisk

Because fdisk_assign_device tries to open block devices with O_EXCL,
when it does it blocks cryptsetup from using partition block devices for
the same disk.

Since we already have a file descriptor for the device, we can just
share it and use fdisk_assign_device_by_fd instead.

## repart: Use blkpg partitions instead of loop devices when possible

We will want to allow future features to keep some devices mounted or
active. So in order to avoid leaving a mess of many loop devices, we can
just already use the partition block device already.

## repart: Rescan disk on failure if we create blkpg partitions on the
fly

Since we did not write the partition table, then the created partitions
should get removed on error.

## repart: Allow keeping luks2 volumes opened

## repart: Add BtrfsReplace=

BtrfsReplace=/mntpnt will move the btrfs filesystem from mount point to
the partition created. After moving, it will resize to take the whole
partition.
This is useful for OS installers that move a live system into a disk and
do not require a reboot.

## repart: Add VolumeName=

When a luks2 device mapper is to be kept alive after execution of
systemd-cryptsetup, the name of the volume will be taken from this
value.

## test: Add test for repart's BtrfsReplace