cal: determine the first day of week from the locale

cal: determine the first day of week from the locale

Previously it defaulted to Sunday rather than using
the value from the locale. Lauri Nurmi <lanurmi@iki.fi>
provided the detailed argument for this change while
Samuel Thibault <samuel.thibault@ens-lyon.org> provided
the information on how to read the first day of the week
from the locale correctly.

[kzak@redhat.com: - fix "cast from pointer to integer",
                    nl_langinfo(_NL_TIME_WEEK_1STDAY) call)]

Signed-off-by: Pádraig Brady <P@draigBrady.com>
Signed-off-by: Karel Zak <kzak@redhat.com>