* callback.c [HAVE_LIMITS_H]: Include limits.h.
Include libiberty.h.
(os_close, os_read, os_write, os_fstat, os_ftruncate): Support fd
being either end of a pipe.
(os_pipe, os_pipe_empty, os_pipe_nonempty): New functions.
(os_shutdown): Clear pipe state.
(default_callback): Initialize new members.