patch 9.0.1374: function for setting options not used consistently

Problem:    Function for setting options not used consistently.
Solution:   Use a function for 'encoding' and terminal options. (Yegappan
            Lakshmanan, closes #12099)
diff --git a/src/proto/optionstr.pro b/src/proto/optionstr.pro
index 3163ba7..43f285a 100644
--- a/src/proto/optionstr.pro
+++ b/src/proto/optionstr.pro
@@ -39,6 +39,7 @@
 char *did_set_diffopt(optset_T *args);
 char *did_set_display(optset_T *args);
 char *did_set_eadirection(optset_T *args);
+char *did_set_encoding(optset_T *args);
 char *did_set_eventignore(optset_T *args);
 char *did_set_fileformat(optset_T *args);
 char *did_set_fileformats(optset_T *args);
@@ -99,6 +100,7 @@
 char *did_set_switchbuf(optset_T *args);
 char *did_set_tabline(optset_T *args);
 char *did_set_tagcase(optset_T *args);
+char *did_set_term_option(optset_T *args);
 char *did_set_termwinkey(optset_T *args);
 char *did_set_termwinsize(optset_T *args);
 char *did_set_termwintype(optset_T *args);