patch 8.1.0863: cannot see what signal caused a job to end

Problem:    Cannot see what signal caused a job to end.
Solution:   Add "termsig" to job_info(). (Ozaki Kiichi, closes #3786)
diff --git a/src/channel.c b/src/channel.c
index cf68271..20cf462 100644
--- a/src/channel.c
+++ b/src/channel.c
@@ -5152,6 +5152,9 @@
     vim_free(job->jv_tty_in);
     vim_free(job->jv_tty_out);
     vim_free(job->jv_stoponexit);
+#ifdef UNIX
+    vim_free(job->jv_termsig);
+#endif
     free_callback(job->jv_exit_cb, job->jv_exit_partial);
     if (job->jv_argv != NULL)
     {
@@ -5908,6 +5911,9 @@
     dict_add_number(dict, "exitval", job->jv_exitval);
     dict_add_string(dict, "exit_cb", job->jv_exit_cb);
     dict_add_string(dict, "stoponexit", job->jv_stoponexit);
+#ifdef UNIX
+    dict_add_string(dict, "termsig", job->jv_termsig);
+#endif
 
     l = list_alloc();
     if (l != NULL)