dehacked: Load Freedoom DEHACKED lump on startup.

If using one of the Freedoom IWADs, detect it by checking for the
FREEDOOM lump, and then load its DEHACKED lump to apply the
cosmetic string changes that it includes. In case we're using an old
version of one of the Freedoom IWADs, don't bomb out with an error
while parsing the DEHACKED lump.