Tweak psql's \connect command to not downcase unquoted database and user

names.  This is a temporary measure to allow backwards compatibility with
7.2 and earlier pg_dump.  7.2.1 and later pg_dump will double-quote mixed
case names in \connect.  Once we feel that older dumps are not a problem
anymore, we can revert this change and treat \connect arguments as normal
SQL identifiers.