install_arch: init pacman keyring on first boot

Creates a systemd service which executes these commands on first boot:

pacman-key --init

pacman-key --populate archlinux

Also it makes sure /etc/pacman.d/gnupg is not absent.

At last it removes /etc/machine-id to make sure systemd launches first
boot services.
2 jobs for pacman_keyring_init in 45 seconds (queued for 7 seconds)