Create functions with common code as a previous step to use them in other parts.
In particular:
- `get_machine_id`
- `get_default_initramfs_image [<kernel_version>]`
- `has_early_microcode <initramfs_image>`
- `get_decompression_command <initramfs_image_header>`

Instead of trying every possible compression method until the right one is
found, parse the header of the initramfs image to get the right one and use it
without hiding possible decompression errors, using the same logic that is
implemented in `lsinitrd.sh`.

…laceholder

Implement and use a new `get_dollar_boot` function to get the `$BOOT` partition
placeholder, as defined in the Boot Loader Specification [1].
If `bootctl` is available, use it to support a possible XBOOTLDR partition,
otherwise, check only the ESP, as before.

[1] https://uapi-group.org/specifications/specs/boot_loader_specification

Use the new functions `get_dollar_boot`, `get_machine_id` and
`get_default_initramfs_image` to get the `outfile` path.

Fixes dracutdevs#1995

Since v6.8-rc1 [1], when the kernel was booted with an initrd and the
`retain_initrd` option is set on the kernel command line, /sys/firmware/initrd
contains the contents of the initrd that the kernel was booted with.

Therefore, there is no need to guess the path of the initrd for `lsinitrd`
and `dracut-initramfs-restore` if this sysfs file exists on the running system.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=2678fd2fe9ee2c569e9cb6b17e786bc8f0753538