patch 9.1.0469: Cannot have buffer-local value for 'completeopt'

Problem:  Cannot have buffer-local value for 'completeopt'
          (Nick Jensen).
Solution: Make 'completeopt' global-local (zeertzjq).

Also for some reason test Test_ColonEight_MultiByte seems to be failing
sporadically now. Let's mark it as flaky.

fixes: #5487
closes: #14922

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/optiondefs.h b/src/optiondefs.h
index 50472e3..7b58f6b 100644
--- a/src/optiondefs.h
+++ b/src/optiondefs.h
@@ -50,6 +50,7 @@
 # define PV_CMS		OPT_BUF(BV_CMS)
 #endif
 #define PV_COM		OPT_BUF(BV_COM)
+#define PV_COT		OPT_BOTH(OPT_BUF(BV_COT))
 #define PV_CPT		OPT_BUF(BV_CPT)
 #define PV_DICT		OPT_BOTH(OPT_BUF(BV_DICT))
 #define PV_TSR		OPT_BOTH(OPT_BUF(BV_TSR))
@@ -654,7 +655,7 @@
 #endif
 			    SCTX_INIT},
     {"completeopt",   "cot",  P_STRING|P_VI_DEF|P_ONECOMMA|P_NODUP,
-			    (char_u *)&p_cot, PV_NONE, did_set_completeopt, expand_set_completeopt,
+			    (char_u *)&p_cot, PV_COT, did_set_completeopt, expand_set_completeopt,
 			    {(char_u *)"menu,preview", (char_u *)0L}
 			    SCTX_INIT},
     {"completepopup", "cpp", P_STRING|P_VI_DEF|P_COMMA|P_NODUP|P_COLON,