We use off_t in the tdb2 interface (for tdb_attribute_flock); we need
to make sure that all callers agree on the size.

This also causes a problem in the tests: it's not enough to include
config.h first, we need the _FILE_OFFSET_BITS define from private.h.
Otherwise, we can disagree about the definitions of F_SETLK, F_SETLKW
and off_t, causing strange problems.