checkoverdues should not require $dbh

checkoverdues should not require $dbh

Passing $dbh around is an ancient style that doesn't know
to use C4::Context.  C4::Context->dbh is efficient, especially
for modules that already use Context, including almost all C4.

I also internalized $today into the SQL using NOW() in the query
and removed sth->finish.  Even though I dislike the return style
that gives the count, then the array, I left it becuase I don't
have time to fix/test all the callers.  However, I did convert
it so it doesn't require a $count variable and its own loop.

Signed-off-by: Galen Charlton <galen.charlton@liblime.com>