    Wait for process completion and return exit status.
    
    Waits for each process specified by a PID and reports its termination status.
    If PID is not given, waits for all currently active child processes,
    and the return status is zero.  PID must be a process ID.
    
    Exit Status:
    Returns the status of the last PID; fails if PID is invalid or an invalid
    option is given.
