short_name_of_{day/month} not compatible with UTF8 chars

[fccm]

short_name_of_day and short_name_of_month use:

String.sub s 0 3

This will only work with languages using ASCII chars only, but it doesn't work with UTF-8 characters.

In addition to the functions refs month_name and day_name we should also create month_short_name and day_short_name.