sim: make nrun the default run program

sim: make nrun the default run program

We want people to stop using the run.c frontend, but it's hard to notice
when it's still set as the default.  Lets flip things so nrun.c is the
default, and users of run.c will get an error by default.  We turn that
error into a warning for existing sims so we don't break them -- this is
mostly meant for people starting new ports.