Disallow bits beyond the mask length for CIDR values, per discussion

Disallow bits beyond the mask length for CIDR values, per discussion
on pghackers.  Arrange for the sort ordering of general INET values
to be network part as major sort key, host part as minor sort key.
I did not force an initdb for this change, but anyone who's running
indexes on general INET values may need to recreate those indexes.