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/proto/insexpand.pro b/src/proto/insexpand.pro
index c9183bb..b10d2a8 100644
--- a/src/proto/insexpand.pro
+++ b/src/proto/insexpand.pro
@@ -44,11 +44,11 @@
 void ins_compl_addleader(int c);
 void ins_compl_addfrommatch(void);
 int ins_compl_prep(int c);
-int set_completefunc_option(void);
+char *set_completefunc_option(void);
 void set_buflocal_cfu_callback(buf_T *buf);
-int set_omnifunc_option(void);
+char *set_omnifunc_option(void);
 void set_buflocal_ofu_callback(buf_T *buf);
-int set_thesaurusfunc_option(void);
+char *set_thesaurusfunc_option(void);
 int set_ref_in_insexpand_funcs(int copyID);
 void f_complete(typval_T *argvars, typval_T *rettv);
 void f_complete_add(typval_T *argvars, typval_T *rettv);