Introduce the world date in 'world'

The 'world' variable now also stores a world date (day, month, year) in
addition to the total days counter world->time.total. These values have
been used (instead of calculate them) in several places. Because of
this, the functions current_month() and current_year() of lclib.c are no
longer needed and have been removed.

Also, a new function calculates the date from the actual number of total
days (for example to inicialize the date when is loaded a saved game).

The global variable months[] is renamed to month_names[].