Bugfix: Use _exit() instead of exit to avoid calling atexit functions (Thanks f8l)

Bugfix: Use _exit() instead of exit to avoid calling atexit functions (Thanks f8l)

Basically, this is the same fix as commit 914ca6cf :-/. Once again, we
called exit() instead of _exit(), but this time it lead to a kill(0,
SIGTERM), effectively killing all processes in the i3 process group,
including i3 itself. The cause for the kill(0) is that nagbar_pid is set
to 0 by fork(), signaling we’re in the child process. The cleanup
handler only checks for nagbar_pid being -1 as a special value, however.