Function Psetaction

* Common function to allow clients to manipulate the action to be taken * on receipt of a signal, receipt of machine fault, entry to a system call, * or exit from a system call. We make use of our private prset_* functions * in order to make this code be common. The 'which' parameter identifies * the code for the event of interest (0 means change the entire set), and * the 'stop' parameter