patch 8.1.1736: viminfo support is spread out

Problem:    Viminfo support is spread out.
Solution:   Move more viminfo code to viminfo.c. (Yegappan Lakshmanan,
            closes #4717)  Reorder code to make most functions static.
diff --git a/src/proto/ex_cmds.pro b/src/proto/ex_cmds.pro
index 783ed11..c31fee3 100644
--- a/src/proto/ex_cmds.pro
+++ b/src/proto/ex_cmds.pro
@@ -34,8 +34,8 @@
 int do_sub_msg(int count_only);
 void ex_global(exarg_T *eap);
 void global_exe(char_u *cmd);
-int read_viminfo_sub_string(vir_T *virp, int force);
-void write_viminfo_sub_string(FILE *fp);
+char_u *get_old_sub(void);
+void set_old_sub(char_u *val);
 void free_old_sub(void);
 int prepare_tagpreview(int undo_sync);
 void ex_help(exarg_T *eap);