o Provide poll_quit callback to simulators
so that they can poll for SIGINT on
clueless OS's.
o Add sim_stop to simulators so that clients
can request a halt (eg gdbtk's STOP button)
Works for PPC!
o Re-arange remote-sim.c so that the
hard work is moved from gdbsim_resume()
to gdbsim_wait() (where it should be).
