configure: add option to set default user

The default user is root by default, which disables root dropping by
default. The user directive or the -u option can still be used to set
the user.

conf.c

@@ -203,7 +203,7 @@ static int lock_memory = 0;
 static char *leapsec_tz = NULL;
 
 /* Name of the user to which will be dropped root privileges. */
-static char *user = NULL;
+static char *user = DEFAULT_USER;
 
 typedef struct {
   NTP_Source_Type type;