patch 9.1.0218: Unnecessary multiplications in backspace code

Problem:  Unnecessary multiplications in backspace code, as
          "col / ts * ts" is the same as "col - col % ts".
Solution: Change "col / ts * ts" to "col - col % ts".  Adjust the loop
          and the comments ins_bs() to be easier to understand.  Update
          tests to reset 'smarttab' properly.
          (zeertzjq)

closes: #14308

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/version.c

@@ -704,6 +704,10 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    218,
+/**/
+    217,
 /**/
     216,
 /**/