patch 8.0.1609: shell commands in the GUI use a dumb terminal

Problem:    Shell commands in the GUI use a dumb terminal.
Solution:   Add the "!" flag to 'guioptions' to execute system commands in a
            special terminal window.  Only for Unix now.
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 6076b03..c025a34 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -7032,7 +7032,7 @@
     rettv->v_type = VAR_JOB;
     if (check_restricted() || check_secure())
 	return;
-    rettv->vval.v_job = job_start(argvars, NULL);
+    rettv->vval.v_job = job_start(argvars, NULL, NULL);
 }
 
 /*