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.