patch 8.0.1795: lose contact with jobs when :gui forks

Problem:    Lose contact with jobs when :gui forks.
Solution:   Don't fork when there is a running job. Make log message for a
            died job clearer.  Also close the terminal when stderr and stdout
            are the same FD.
diff --git a/src/proto/channel.pro b/src/proto/channel.pro
index 2505df5..8d26158 100644
--- a/src/proto/channel.pro
+++ b/src/proto/channel.pro
@@ -54,6 +54,7 @@
 int get_job_options(typval_T *tv, jobopt_T *opt, int supported, int supported2);
 channel_T *get_channel_arg(typval_T *tv, int check_open, int reading, ch_part_T part);
 void job_free_all(void);
+int job_any_running(void);
 int win32_build_cmd(list_T *l, garray_T *gap);
 void job_cleanup(job_T *job);
 int set_ref_in_job(int copyID);