Implemented an earliest booking date using absolute dates, controlled by the new config variables $min_booking_date_enabled and $min_booking_date

git-svn-id: http://svn.code.sf.net/p/mrbs/code/mrbs/trunk@2791 25f7e783-145e-4f2d-a9f8-dd7182bfe9c7
Corrected error in comment

git-svn-id: http://svn.code.sf.net/p/mrbs/code/mrbs/trunk@2792 25f7e783-145e-4f2d-a9f8-dd7182bfe9c7