Clarify meaning of sim_signalled's SIGRC argument. Document that this

Clarify meaning of sim_signalled's SIGRC argument.  Document that this
isn't possible in sim-reason.c and just return the target SIGRC
instead.

For simulators that rely on sim-reason.c, replace SIG* with SIM_SIG*.

Hack nrun.c so that when it is executed (ARGV[0]) as `step' instead
of `run' it single steps the simulator.  Allows testing of single step
without full GDB.