tdb: make TDB_NOSYNC merely disable sync.

tdb: make TDB_NOSYNC merely disable sync.

(As suggested by Stefan Metzmacher, based on the change to ntdb.)

Since commit ec96ea690edbe3398d690b4a953d487ca1773f1c, we handle the case
where a process dies during a transaction commit.  Unfortunately, TDB_NOSYNC
means this no longer works, as it disables the recovery area as well as the
actual msync/fsync.  We should do everything except the syncs.

This also means we can do a complete test with $TDB_NO_FSYNC set; just
to get more complete coverage, we disable it explicitly for one test
(where we override the actual sync calls anyway).

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