keepcounter serial option

A "keepcounter" option for the serial would try to use the input serial (if it is after the current output serial), before resorting to using current output serial + 1.

This is an alternative to "keep" which allows the output serial to be the same as the input serial when the zone is changed, but unlike "keep" does not require manual intervention when a subsequent signature change requires a new serial.

The expected use for this would be a time-based input serial which is likely to increment faster than the signed zone.

e.g. Input 2009092200 and Output 2009092200, 2009092201, 2009092202 (signature changes) followed by Input 2009093000 which results in Output 2009093000.