Add warnings about debugging mode

Debugging mode is intended to enable administrative commands which
are especially unsafe and could even be used to run arbitrary shell
commands or alter files accessible to the user under which the
service runs. Log a warning about it at startup if the option has
been set, and insert some comments to that effect in the sample
configuration so that hopefully no one will turn it on without due
forethought. Also add a mention about its risks in the config doc.