reduce friction in auto-upgrade process

[majewsky]

Minimize number and length of user interactions where appropriate.

• stay silent if no updates and user (i.e., me) has updated the system on that day already (i.e., has a line =~ /\[PACMAN] starting full system upgrade dated to same day in pacman.log)
• install non-critical updates without confirmation (i.e., pacman -Su --ignore $pkgs, with packages configured in e.g. /etc/auto-upgrade/*.conf; default being linux and linux-headers), then ask interactively about pacman -Su without --ignore if something left
• make a guess about when to restart services, by looking at which unit files were installed by the upgraded packages