Support running as a regular (non-root) user

Running the whole script as root is excessive and it tremendously complicates signing the Squashfs image with GPG. It would be better to elevate to support running as a regular using, e.g. using unshare.

TODO:

• !58 (merged): Add -rational-rock to xorriso command, so that all files in the ISO (in Rock Ridge, not Squashfs) are owned by root and have sane permissions.
• Launch commands that require root privileges (i.e. manipulate files in $pacstrap_dir) using unshare when the script is run as a regular user.

The things that require root are:

• _make_custom_airootfs: file copying to airootfs and chown.
• _make_packages: pacstrap
• _make_customize_airootfs: /etc/skel copying to airootfs and arch-chroot for customize_airootfs.sh.
• _make_pkglist: pacman --sysroot - to generate packages list.
• _make_boot_uefi-x64.systemd-boot.esp: mount - to mount the FAT file system image.
• _cleanup: find ... -delete on airootfs and workdir. Also the printf that creates an empty /etc/machine-id.
• _mkairootfs_ext4+squashfs, _mount_airootfs, _umount_airootfs: mount and chown with airootfs_image_type="ext4+squashfs".
• _run_mksquashfs: mksquashfs - it needs full access to airootfs to create the squashfs image.