Libhdfs backport

Backported hadoop 2.6.0  libhdfs imlementation to all 2.X . 


Libhdfs version 2.6.0 has many generic improvements, e.g. better error handling and minor error fixes that appear to be independent of the specific hadoop version used.
`HADOOP_LIBHDFS_V2` does not appear anywhere in the C code. Moreover, this assignment is overridden two lines from here