sim: split program path out of argv vector

We use the program argv to both find the program to run (argv[0]) and
to hold the arguments to the program.  Most of the time this is fine,
but if we want to let programs specify argv[0] independently (which is
possible in standard *NIX programs), this double duty doesn't work.

So let's split the path to the program to run out into a separate
field by itself.  This simplifies the various sim_open funcs too.

By itself, this code is more of a logical cleanup than something that
is super useful.  But it will open up customization of argv[0] in a
follow up commit.  Split the changes to make it easier to review.

sim/common/nrun.c

@@ -103,7 +103,7 @@ main (int argc, char **argv)
   if (prog_argv == NULL || *prog_argv == NULL)
     usage ();
 
-  name = *prog_argv;
+  name = STATE_PROG_FILE (sd);
 
   /* For simulators that don't open prog during sim_open() */
   if (prog_bfd == NULL)