[9.18] chg: dev: Use _exit() in the fatal() function

Since the fatal() isn't a correct but rather abrupt termination of the
program, we want to skip the various atexit() calls because not all
memory might be freed during fatal() call, etc.  Using _exit() instead
of exit() has this effect - the program will end, but no destructors or
atexit routines will be called.

Backport of MR !8703

Merge branch 'backport-ondrej/use-_exit-in-fatal-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!9263