* linux-nat.c (linux_nat_attach, linux_nat_detach): Make attach/

* linux-nat.c (linux_nat_attach, linux_nat_detach): Make attach/
detach work asynchronously.

* inf-ptrace.c (inf_ptrace_attach): Ditto.

* infcmd.c (attach_command): Ditto.

* inf-child.c (inf_child_target): Reset to_terminal_inferior/
to_terminal_ours to synchronous methods.

* async-nat-inferior.c (gdb_process_events): Don't poll for
exceptions.
(gdb_process_pending_event): Use pid_to_ptid.
(gdb_inferior_reset): Remove last_thread component.

* async-nat-inferior.h (struct gdb_inferior_status): Remove
last_thread component.