define default values in schema to avoid errors from MySQL >= 5.6

see https://blogs.oracle.com/supportingmysql/entry/mysql_server_5_6_default

as of 5.6 (including for Percona variants, notably XtraDB Cluster, which is what I'm using), STRICT_TRANS_TABLES is now the default setting in /etc/my.cnf (effectively, "use strict;" for MySQL). one consequence of this is that some schemas which were previously valid (albeit not recommended, and would produce warnings) will now produce fatal errors instead. In this case, the fix was just to add default values where the error messages indicated (the "DROP TABLE IF EXISTS nt_nameserver_export_types" addition was because I got an error to that effect from running the create_tables.pl script multiple times; this patch brings that SQL statement in line with the others, and makes the script idempotent).