defer some daemon setup routines

A first step towards untangling some of the intermingled data and
functionality setup tasks for the daemons:

Daemon::new now only validates and untaints arguments, but doesn't
perform any actions such as setuid/setgid until the new Daemon::setup
method which is now executed from Daemon::start right before entering
Daemon::$server_run.

CLIHandler::handle_cmd now takes an optional $preparefunc which is
called after handling 'printmanpod' and 'verifyapi'.