patch 9.0.0003: functions are global while they could be local

Problem:    Functions are global while they could be local.
Solution:   Add "static".  Add a few tests. (Yegappan Lakshmanan,
            closes #10612)
diff --git a/src/userfunc.c b/src/userfunc.c
index 9b960b7..9f84422 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -32,6 +32,7 @@
 static void funccal_unref(funccall_T *fc, ufunc_T *fp, int force);
 static void func_clear(ufunc_T *fp, int force);
 static int func_free(ufunc_T *fp, int force);
+static char_u *untrans_function_name(char_u *name);
 
     void
 func_init()
@@ -4073,7 +4074,7 @@
  * This can be used to first search for a script-local function and fall back
  * to the global function if not found.
  */
-    char_u *
+    static char_u *
 untrans_function_name(char_u *name)
 {
     char_u *p;