patch 8.1.2145: cannot map <C-H> when modifyOtherKeys is enabled

Problem:    Cannot map <C-H> when modifyOtherKeys is enabled.
Solution:   Add the <C-H> mapping twice, both with modifier and as 0x08.  Use
            only the first one when modifyOtherKeys has been detected.
diff --git a/src/option.c b/src/option.c
index 60c1141..898068b 100644
--- a/src/option.c
+++ b/src/option.c
@@ -4495,7 +4495,7 @@
     {
 	--arg;			    /* put arg at the '<' */
 	modifiers = 0;
-	key = find_special_key(&arg, &modifiers, TRUE, TRUE, FALSE);
+	key = find_special_key(&arg, &modifiers, TRUE, TRUE, FALSE, TRUE, NULL);
 	if (modifiers)		    /* can't handle modifiers here */
 	    key = 0;
     }