[CHEF-3367] prevent accidental removal of chef-client daemon pidfile whe...

...n child processes of the daemon exit

Children forked from the chef-client daemon (such as those triggered
during runs of only_if commands) inherit the at_exit handler of the
daemon, which removes the daemon's pid file.  remove_pid_file now
only removes the pid file if it's invoked by the same process which
created it in the first place.

http://tickets.opscode.com/browse/CHEF-3367