patch 8.2.0863: cannot set a separate color for underline/undercurl

Problem:    Cannot set a separate color for underline/undercurl.
Solution:   Add the t_AU and t_8u termcap codes. (Timur Celik, closes #6011)
diff --git a/src/optiondefs.h b/src/optiondefs.h
index 374764b..5ed9be9 100644
--- a/src/optiondefs.h
+++ b/src/optiondefs.h
@@ -2924,6 +2924,7 @@
 
     p_term("t_AB", T_CAB)
     p_term("t_AF", T_CAF)
+    p_term("t_AU", T_CAU)
     p_term("t_AL", T_CAL)
     p_term("t_al", T_AL)
     p_term("t_bc", T_BC)
@@ -3002,6 +3003,7 @@
     p_term("t_ZR", T_CZR)
     p_term("t_8f", T_8F)
     p_term("t_8b", T_8B)
+    p_term("t_8u", T_8U)
 
 // terminal key codes are not in here