Print warning when falling back to increment soa serial method

Print warning when falling back to increment soa serial method

When using the `unixtime` or `date` method to update the SOA serial,
`named` and `dnssec-signzone` would silently fallback to `increment`
method to prevent the new serial number to be smaller than the old
serial number (using the serial number arithmetics).  Add a warning
message when such fallback happens.