* linux-nat.c (debug_linux_nat_async): Delete.

	* linux-nat.c (debug_linux_nat_async): Delete.
	Replace all references to use debug_linux_nat instead.
	(show_debug_linux_nat_async): Delete.
	(sigchld_handler): Call ui_file_write_async_safe instead of
	fprintf_unfiltered.
	(_initialize_linux_nat): Remove `set debug lin-lwp-async'.
	* ui-file.c (struct ui_file): New member to_write_async_safe.
	(null_file_write_async_safe): New function.
	(ui_file_write_async_safe): New function.
	(set_ui_file_write_async_safe): New function.
	(ui_file_new): Initialize to_write_async_safe.
	(stdio_file_write_async_safe): New function.
	(struct stdio_file): New member fd.
	(stdio_file_new): Initialize to_write_async_safe, fd.
	(stdio_file_read, stdio_file_isatty): New stdio->fd instead of calling fileno.
	* ui-file.h (ui_file_write_async_safe_ftype): New typedef.
	(set_ui_file_write_async_safe): Declare.
	(ui_file_write_async_safe): Declare.

	doc/
	* gdb.texinfo (Completion): Update example.
	(Debugging Output): Delete `set/show debug lin-lwp-async'.