Add uselinkdelays boolean to experiments table, to be set in the NS

file with a tb- command and used in assign_wrapper to use link delays
instead of delay nodes. Default is zero, of course.

sql/database-create.sql

@@ -197,6 +197,7 @@ CREATE TABLE experiments (
   canceled tinyint(4) NOT NULL default '0',
   batchstate varchar(12) default NULL,
   event_sched_pid int(11) default '0',
+  uselinkdelays tinyint(4) NOT NULL default '0',
   uselatestwadata tinyint(4) NOT NULL default '0',
   usewatunnels tinyint(4) NOT NULL default '1',
   wa_delay_solverweight float default '0',