patch 9.1.0218: Unnecessary multiplications in backspace code

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>