PID dir can be specified for worker processes

Added command-line argument that allows the PID folder to be specified for the worker processes.

(Why?  Even though a user can launch multiple god processes, all of them write pids to the same ~/.god/pids directory.  Thus, each god process can clobber the PID files for the other god processes.  Specifying a separate --managed_pid_dir for each god process ensures that they will not clobber one another)