bugfix: all fixes to ttl value were lost

In the first lines of the function `updateBind()` you read `ttl` value from the configuration, then some sanity checks are performed:
  * the value must be int
  * the value must be at least 300

The problem is that when it get written to create the actual `nsupdate` command, the value is taken again from the original configuration loosing all the performed sanity checks.