dehacked: Add ZDoom obituaries.

When playing in ZDoom-based ports, a message is shown when the player
dies which depends on the monster that killed them or (in deathmatch)
the weapon they were killed with. Because ZDoom uses obituaries that
mention the Doom weapons and monsters, override so that Freedoom is
internally consistent when playing using these ports.
dehacked: Change SKAG 1337 obituary messages.

At Protox's suggestion, change the SKAG obituary messages. These
now look like: "fraggle had his ass kicked by Protox's SKAG!"
dehacked: Add startup message.

Boom added the ability to show up to 5 text strings on startup. Use
this to show some Freedoom licensing blurb and link to the website.