doom: Refactor IWAD dehacked patch loading.

As per suggestions from Fabian Greffrath:
 * Change -noiwaddeh to -nodeh for consistency with other source ports
   (Boom-derived source ports use this to disable loading of DEHACKED
   lumps).
 * Extend the parameter so that it also disables loading of the Chex
   Quest dehacked patch.
 * Refactor the code for loading IWAD dehacked patches to all be in a
   single function.