import: add new "--direct" mode + add controls for turning certain features on/off

import: add new "--direct" mode + add controls for turning certain features on/off

This reworks/modernizes the tar/raw import logic and adds the following
new features:

- Adds the ability to control btrfs subvol and quota behaviour which was
  previously always on via an env var and cmdline arg

- Adds control whether to sync() stuff after writing it, similar via env
  var + cmdline arg

- Similar, the QCOW2 unpacking logic that was previously the implied
  default may now be controlled via env var + cmdline arg.

- adds a "direct" mode. In this mode, the systemd-import tool can be
  used as a simple tool for decompressing/unpacking/installing arbitrary
  files, without all the additional meta data and auxiliary resources,
  i.e.  outside of the immediate disk image context. Via the new
  --offset= and --size-max= switches the downloaded data can be written
  to specific locations of a file (which is particularly useful to use
  the tool to download fs images and write them to a partition location
  before actually creating the partition).

We'll later use the latter feature for "sysupdate" concept, where images
can be directly be written to partitions. That way the systemd-import
binary will be used as backend for both "systemd-importd" and
"systemd-sysupdate" and share most of the same code.