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/version.c b/src/version.c
index 7e12ce7..5f4e2c8 100644
--- a/src/version.c
+++ b/src/version.c
@@ -767,6 +767,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1609,
+/**/
     1608,
 /**/
     1607,