patch 9.0.1262: the did_set_string_option function is too long

Problem:    The did_set_string_option function is too long.
Solution:   Split off functionality to individual functions.  (Yegappan
            Lakshmanan, Lewis Russell, closes #11904)
diff --git a/src/gui_xim.c b/src/gui_xim.c
index 163cbda..86273b5 100644
--- a/src/gui_xim.c
+++ b/src/gui_xim.c
@@ -73,16 +73,22 @@
 static callback_T imaf_cb;	    // 'imactivatefunc' callback function
 static callback_T imsf_cb;	    // 'imstatusfunc' callback function
 
-    int
+    char *
 set_imactivatefunc_option(void)
 {
-    return option_set_callback_func(p_imaf, &imaf_cb);
+    if (option_set_callback_func(p_imaf, &imaf_cb) == FAIL)
+	return e_invalid_argument;
+
+    return NULL;
 }
 
-    int
+    char *
 set_imstatusfunc_option(void)
 {
-    return option_set_callback_func(p_imsf, &imsf_cb);
+    if (option_set_callback_func(p_imsf, &imsf_cb) == FAIL)
+	return e_invalid_argument;
+
+    return NULL;
 }
 
     static void