makefs: Record a larger TXG number in the uberblock

By default, OpenZFS will perform metadata verification of the most
recent TXGs, but this can be very slow since all data in a pool
generated by makefs was logically written in a single transaction.

Avoid triggering this verification by default, but add an option to
restore the previous behaviour and enable it in regression test cases.

Reported by: cperciva
Tested by: cperciva (previous version)
MFC after: 2 weeks