[Simh] RSTS/E problem in 3.9

[Bob Supnik]

Thanks for finding this before final release of 3.9.

In pdp11_io.c, the array of constants defining internal interrupts is 
backwards.  It should be:

static const int32 int_internal[IPL_HLVL] = {
     0,             INT_INTERNAL1, INT_INTERNAL2, INT_INTERNAL3,
     INT_INTERNAL4, INT_INTERNAL5, INT_INTERNAL6, INT_INTERNAL7
     };

With this fix, RSTS/E boots correctly and also seems to idle correctly.  
Note that there is a 10-20 second lag before idling will "kick in" in 
the current release.

/Bob Supnik