New post_crash_dump_hook

Run it from crash_dump().  Useful to move core files out of the way,
alert deities by e-mail, and so forth.

include/server.h

@@ -49,6 +49,7 @@ void update_main(void);
 void update_init(void);
 int update_trigger(void);
 int update_reschedule(void);
+int run_hook(char *, char *);
 int shutdown_initiate(int);
 
 /* thread entry points */