#12 Patch to update to increment SOA serial number

[Howard Wilkinson]

This is a patch to the update function in mydns-1.1.0 that provides support for the incrementing of the serial number in the SOA as part of the update transaction.

The patch mimics the code in the admin.php page in deciding how the serial number is encoded.

Thus it allows the following:
o a simple incrementing serial number
o a unix timestamp
o an encoding of the current year-month-day-serial

I have coded this to roll over to the next day if the updates are too frequent in one day. Roll round at 2**32 is not handled cleanly at present but I will fix this later if it is an issue.

This patch is applied after my clean up patch and the recursive fix patch already applied.

[Howard Wilkinson]

Fix update to increment serial number