patch 9.0.0706: :help in a narrow window always opens at the top

Problem:    :help in a narrow window always opens at the top.
Solution:   Respect 'splitbelow'. (closes #11319)
diff --git a/src/help.c b/src/help.c
index 9dbd256..4d1efe8 100644
--- a/src/help.c
+++ b/src/help.c
@@ -150,7 +150,7 @@
 	    n = WSP_HELP;
 	    if (cmdmod.cmod_split == 0 && curwin->w_width != Columns
 						  && curwin->w_width < 80)
-		n |= WSP_TOP;
+		n |= p_sb ? WSP_BOT : WSP_TOP;
 	    if (win_split(0, n) == FAIL)
 		goto erret;