tail: Fix operation with +n argument

POSIX says

  Copying shall begin at the point in the file indicated by the -c
  number or -n number options.

  The origin for counting shall be 1; that is, -c +1 represents the
  first byte of the file, -c -1 the last.

  The origin for counting shall be 1; that is, -n +1 represents the
  first line of the file, -n -1 the last.
1 file changed