ntdb: create initial database to be multiple of NTDB_PGSIZE.

ntdb: create initial database to be multiple of NTDB_PGSIZE.

As copied from tdb1, there is logic in the transaction code to handle
a non-PGSIZE multiple db, but in fact this only happens for a
completely unused database: as soon as we add anything to it, it is
expanded to a NTDB_PGSIZE multiple.

If we create the database with a free record which pads it out to
NTDB_PGSIZE, we can remove this last-page-is-different logic.

Of course, the fake ntdbs we create in our tests now also need to be
multiples of NTDB_PGSIZE, so we change some numbers there too.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>