dehacked: Change Freedoom monster names.

Some of these names do not match the sprites that we currently have
in the IWADs. Change to be more fitting names. Remove "hell" or
"undead" references as it's not clear that Freedoom's story has a
hell theme, and use "minigun" instead of "mini-gun".
dehacked: Change intermission/story text.

Use new "Phase 2" name and just refer to "episode n" rather than
"Freedoom episode n". Don't refer to "the elite guard" as we don't
have a story or know who they are, and give MAP07/MAP12 names based
on the text. Rewrite C6TEXT entirely to match the current MAP31/MAP32.