tdb2: unify tdb1_open into tdb_open

tdb2: unify tdb1_open into tdb_open

Finally, we gut tdb1_open() to the tdb1-specific parts, and call it
from tdb_open if they specify the TDB_VERSION1 flag or the version is
a TDB1.

We also unify tdb_close(), based on the TDB_VERSION1 flag.

Note that tdb_open(TDB_VERSION1) will fail on an existing tdb if it's
a TDB2.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
(Imported from CCAN commit c8c3b3568677e8b0105f84e4ab068c580faf4591)