unit: allow extension of unit files with .d/*.conf drop-ins

For all unit files foobar.service we will now read
foobar.service.d/*.conf, too. This may be used to override certain unit
settings without having to edit unit files directly.

This makes it really easy to change specific settings for services
without having to edit any unit file:

mkdir /etc/systemd/system/avahi-daemon.service.d/
echo -e '[Service]\nNice=99' > /etc/systemd/system/avahi-daemon.service.d/nice.conf
systemctl daemon-reload
unit: instead of directly loading drop-in configuration snippets use conf_files_list_strv()

This has the benefit of allowing the usual overriding/masking knowledge
everybody loves so much.