libpqxx 7.0.3: Important large objects fix

This new version fixes an important bug which could happen when reading large objects through the ilostream class. On systems where char is a signed type (which I believe includes most desktop systems), if the stream found any byte with value -1 at a buffer boundary, then it would mistake that byte for an end-of-file marker.

Thanks to @tomlankhorst for finding and fixing this bug. A similar update for the 6.4 series is forthcoming.

There are also some other fixes: large-object streams open in binary mode. Some compile errors have been fixed (though the tests may not pass) on non-Unicode systems, and in some scenarios, lost database connections are now properly reported as broken_connection exceptions instead of sql_error exceptions. Unfortunately this does not cover all scenarios, so more work will be needed.