Revision 4904

Preliminary commit of code to restrict access to assignments by client
IP address.  This commit adds the database tables for IP address
restriction, updates check_modules.pl to require Net::IP for address
matching, adds database calls in DB.pm for manipulating locations and
addresses, and adds location restriction to the set editing in
ProblemSetDetail.

This commit does not add actual restriction to sets, nor the ability
to create locations to which to restrict access.