lib/tdb: wean off TDB_ERRCODE.

lib/tdb: wean off TDB_ERRCODE.

It was a regrettable hack which I used to reduce line count in tdb; in fact it caused confusion as can be seen in this patch.
In particular, ecode now needs to be set before TDB_LOG anyway, and having it exposed in
the header is useless (the struct tdb_context isn't defined, so it's doubly useless).
Also, we should never set errno, as io.c was doing.

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