fix crashes in kdeinit5 when ddjvu is missing
The child process tries to call ddjvu, but if this is missing, it exits with "exit(1)" which calls the atexit registered code, and tears down the kdeinit5 process.
The correct way is to use "_exit(1)" and not do atext code.
Fixes #420074 https://bugs.kde.org/show_bug.cgi?id=420074