Fix atexit bug for forked-checkpoint.

The child process inherits the atexit() handlers of its parent process.
Thus we must use _exit() instead of exit() so that the atexit() handlers
are not popped when the child exit.

git-svn-id: svn://svn.code.sf.net/p/dmtcp/code/trunk@2764 ba6dc0ae-232f-0410-883f-9ebac64fc565