patch 8.0.0957: a terminal job can deadlock when sending many keys

Problem:    When term_sendkeys() sends many keys it may get stuck in writing
            to the job.
Solution:   Make the write non-blocking, buffer keys to be sent.
diff --git a/src/proto/channel.pro b/src/proto/channel.pro
index 8989116..f9a7014 100644
--- a/src/proto/channel.pro
+++ b/src/proto/channel.pro
@@ -35,6 +35,7 @@
 void common_channel_read(typval_T *argvars, typval_T *rettv, int raw);
 channel_T *channel_fd2channel(sock_T fd, ch_part_T *partp);
 void channel_handle_events(void);
+void channel_set_nonblock(channel_T *channel, ch_part_T part);
 int channel_send(channel_T *channel, ch_part_T part, char_u *buf, int len, char *fun);
 void ch_expr_common(typval_T *argvars, typval_T *rettv, int eval);
 void ch_raw_common(typval_T *argvars, typval_T *rettv, int eval);