cal: fix days of week alignment for year view (thanks igor@9lab.org)

cal: fix days of week alignment for year view (thanks igor@9lab.org)

  term% cal -s1 2021
                                  2021

           Jan                    Feb                    Mar
   M Tu  W Th  F Sa Su     M Tu  W Th  F Sa Su     M Tu  W Th  F Sa Su
               1  2  3    1  2  3  4  5  6  7    1  2  3  4  5  6  7
   4  5  6  7  8  9 10    8  9 10 11 12 13 14    8  9 10 11 12 13 14
  11 12 13 14 15 16 17   15 16 17 18 19 20 21   15 16 17 18 19 20 21
  18 19 20 21 22 23 24   22 23 24 25 26 27 28   22 23 24 25 26 27 28
  25 26 27 28 29 30 31                          29 30 31
  …

Note how the days (i.e. ' M Tu  W Th  F Sa Su') for Feb and Mar
do not align with the day numbers.

This is because an extra space is left *before* adding the terminating
'\0' via the pointer `dayw`.

With the patch applied the calendar aligns nicely for the year view:

  term% cal -s1 2021
                                  2021

           Jan                    Feb                    Mar
   M Tu  W Th  F Sa Su    M Tu  W Th  F Sa Su    M Tu  W Th  F Sa Su
               1  2  3    1  2  3  4  5  6  7    1  2  3  4  5  6  7
   4  5  6  7  8  9 10    8  9 10 11 12 13 14    8  9 10 11 12 13 14
  11 12 13 14 15 16 17   15 16 17 18 19 20 21   15 16 17 18 19 20 21
  18 19 20 21 22 23 24   22 23 24 25 26 27 28   22 23 24 25 26 27 28
  25 26 27 28 29 30 31                          29 30 31
  …