Make sure target supports non-stop.

	Make sure target supports non-stop.
        * infcmd.c (run_command_1, attach_command): If non-stop mode
        is requested, verify the target supports it.
        * linux-nat.c (linux_nat_supports_non_stop): New.
        (linux_nat_add_target): Register the above.
        * target.c (find_default_supports_non_stop)
        (target_supports_non_stop): New.
        (init_dummy_target): Register find_default_supports_non_stop.
        * target.h (struct target_ops): New field to_supports_non_stop.
        (target_supports_non_stop): New.