[server] Start ChildProcessManager's thread after the fork.

Previously hatoholInit() starts ChildProcessManager's thread to
collect child processes before the fork. As a result, the thread
isn't after the fork and child processes remain in zombie state.

This patch start it after the fork if hatohol runs in deamon mode.
[server] Always start ChildProcessManager's thread even if foreground mode.
Merge pull request #727 from project-hatohol/fix-child-processes-not-collected

Fix child processes not collected