mood.c: Improve log output for current mood.

When a new mood is loaded, we print some information about the score
table, like the number of admissible files, the average last_played
and num_played values, and the empiric standard deviation of these
quantities. However, since last_played is measured in seconds after
the epoch, the reported numbers are rather large.

This commit changes log_statistics() of mood.c to report the mean
value and the standard deviation in number of days.

Since loading a new mood happens not very frequently, let's increase
the severity of these log messages from INFO to NOTICE. If the new
mood has no admissible files we now log the message as a warning
rather than with severity NOTICE.