patch 8.1.1966: some code in options.c fits better elsewhere

Problem:    Some code in options.c fits better elsewhere.
Solution:   Move functions from options.c to other files. (Yegappan
            Lakshmanan, closes #4889)
diff --git a/src/proto/map.pro b/src/proto/map.pro
index 3aa93e9..05d66fb 100644
--- a/src/proto/map.pro
+++ b/src/proto/map.pro
@@ -19,6 +19,9 @@
 void get_maparg(typval_T *argvars, typval_T *rettv, int exact);
 void init_mappings(void);
 void add_map(char_u *map, int mode);
+int langmap_adjust_mb(int c);
+void langmap_init(void);
+void langmap_set(void);
 void ex_abbreviate(exarg_T *eap);
 void ex_map(exarg_T *eap);
 void ex_unmap(exarg_T *eap);