Implement Sys_ReLaunch() for Linux, refactor it

It now works on Linux so executing it doesn't freeze the game
like described in
https://github.com/RobertBeckebans/RBDOOM-3-BFG/issues/33

Furthermore, this function doesn't have any parameters anymore
(on any platform) because the only thing supplied was the original
program arguments +"+set com_skipIntroVideos 1" anyway - this is now
done in Sys_ReLaunch() (also on Windows).
Having the program arguments as one string is bad on Linux/POSIX
because there it's expected that the program arguments are one
C-string per argument.