* target.c (target_async_permitted, target_async_permitted_1)
(set_maintenance_target_async_permitted)
(show_maintenance_target_async_permitted): New.
(initialize_targets): Register 'set target-async'.
* target.h (target_async_permitted): Declare.
* linux-nat.c (linux_nat_async_enabled)
(linux_nat_async_permitted, set_maintenance_linux_async_permitted)
(show_maintenance_linux_async_permitted): Remove.
(sigchld_handler, linux_nat_is_async_p, linux_nat_can_async_p)
(get_pending_events, linux_nat_async): Use target_async_permitted.
(linux_nat_set_async_mode): Remove, moving the only used bits
into...
(linux_nat_setup_async): This.
(_initialize_linux_nat): Do not register 'maint set linux-async'.
Use linux_nat_setup_async.
* remote.c (remote_async_permitted, remote_async_permitted_set)
(set_maintenance_remote_async_permitted)
(show_maintenance_remote_async_permitted): Remove.
(remote_open_1, remote_terminal_inferior, remote_can_async_p)
(remote_is_async_p): Use target_async_permitted.
(_initialize_remote): Don't register 'main set remote-async'.
* mi/mi-cmds.c (mi_cmds): Register -list-target-features.
* mi/mi-cmds.h (mi_cmd_list_target_features): New.
* mi/mi-main.c (mi_cmd_list_target_features): New.
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