Change the default for os_info.OS back to 'Unknown' instead of NULL.

Found by Tim.

sql/database-create.sql

@@ -1268,7 +1268,7 @@ CREATE TABLE os_info (
   creator varchar(8) default NULL,
   created datetime default NULL,
   description tinytext NOT NULL,
-  OS enum('Unknown','Linux','FreeBSD','NetBSD','OSKit','Windows','Other') NOT NULL default 'Unknown',
+  OS enum('Unknown','Linux','FreeBSD','NetBSD','OSKit','Windows','TinyOS','Other') default 'Unknown',
   version varchar(12) default '',
   path tinytext,
   magic tinytext,

sql/database-migrate.txt

@@ -2451,3 +2451,7 @@ last_net_act,last_cpu_act,last_ext_act);
 		unsigned default NULL after panic_date;
         alter table experiment_resources add delay_capacity tinyint(3) \
 		unsigned default NULL after maxlinks;
+
+1.324: Change the default for os_info.OS back to 'Unknown' instead of NULL:
+
+         alter table os_info alter column OS set default 'Unknown';