Instead of setting User/Group only when DynamicUser is disabled, the
previous version of the code set it only when it was enabled. This
caused services with DynamicUser enabled to actually run as nobody, and
services without DynamicUser enabled to run as root.
Regression from fbb7e0c82f.
Use nixos-fw chain instead of INPUT so that the rules don't keep
stacking everytime the firewall is reloaded.
This also adds a comment to each rule about the associated exporter.
- prometheus exporters are now configured with
`services.prometheus.exporters.<name>`
- the exporters are now defined by attribute sets
from which the options for each exporter are generated
- most of the exporter definitions are used unchanged,
except for some changes that should't have any impact
on the functionality.
