libhdfs++: Fix inconsistencies with libhdfs C API

There is at least 1 minor inconsistency with the libhdfs api. Currently hdfsSeek returns a 32 bit int to indicate the result offset of a seek, similar to a posix lseek. It should return 1 and 0 as error codes. If someone starts using hdfsSeek like they would lseek everything works great - until you have an >4GB offset and the upper bits are truncated.