Echo java process's parent pid to the pid file intermediate state

In hadoop-function.sh file，there is hadoop_start_daemon and hadoop_start_daemon_wrapper functions.

hadoop_start_daemon_wrapper invoke hadoop_start_daemon and put it to background.

In  hadoop_start_daemon function, echo $$ > pidfile，cause this scenario

because hadoop_start_daemon is in a subshell by ampersand, and $ expands to the process ID of the current shell, not the subshell.