games-server/ut2004-ded: EAPI 7, drop games eclass, misc improvements

games-server/ut2004-ded: EAPI 7, drop games eclass, misc improvements

This now blocks games-server/ut2004(-data) entirely rather than just
the dedicated USE flag. This allows both to be installed to
/opt/ut2004, vastly simplying the related ebuilds.

This was admittedly done in a hurry but it seems to work. I have
chosen nobody:nobody for the server user and group because I don't
have time to formally request one. This may be sufficient anyway.

Also fixes collision with games-fps/ut2004-bonuspack-ece.

Closes: https://bugs.gentoo.org/594642
Package-Manager: Portage-2.3.78, Repoman-2.3.17
Signed-off-by: James Le Cuirot <chewi@gentoo.org>