patch 9.0.0606: system() opens a terminal window when "!" is in 'guioptions'

Problem:    system() opens a terminal window when using the GUI and "!" is in
            'guioptions'.
Solution:   Do not use a terminal window when the SHELL_SILENT flag is used.
            (closes #11202)
diff --git a/src/os_unix.c b/src/os_unix.c
index 1ac6e19..e86e308 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -5476,7 +5476,8 @@
     ch_log(NULL, "executing shell command: %s", cmd);
 #endif
 #if defined(FEAT_GUI) && defined(FEAT_TERMINAL)
-    if (gui.in_use && vim_strchr(p_go, GO_TERMINAL) != NULL)
+    if (gui.in_use && vim_strchr(p_go, GO_TERMINAL) != NULL
+					      && (options & SHELL_SILENT) == 0)
 	return mch_call_shell_terminal(cmd, options);
 #endif
 #ifdef USE_SYSTEM