Seeding the Fortuna PRNG for the first time
You have to appreciate that Bruce and Niels' paper isn't source code. They wrote "We assume there are several sources of entropy in the environment" and "feel free to
choose AES (Rijndael), Serpent, or Twofish for this function".  It's a bit of a concept paper without a hard implementation.  That's been left to others.

An example of this is on Silicon Labs micro controllers.  That implementation is ultimately seeded with real entropy from reading the internal temperature sensor.  Those tend to jitter somewhat, creating true randomness. Even they though add this proviso: "The user is encouraged to implement additional entropy
sources."  The implementation of FreeBSD palms this off to various device drivers to call the random_harvest_*() function themselves.  

This entropy input step is outside of the Fortuna construct. They clearly couldn't predict where Fortuna would be running so it was correct to leave this to particular implementations.  It could easily be avalanche diodes, keyboard key scanning firmware or even the internal temperature sensors that are scattered throughout desktop motherboards these days.