features/compress: Added writev

Writev compresses data on client and decompresses on server.
Similarly readv compresses data on server and decompresses on client.

The possible values for "mode" option of the cdc xlator were compress/decompress earlier.
This has been changed after adding writev. Currently the mode option can be client/server.

Other minor changes :
* Check for min file size and buffer size while loading xlator
* Change OS type in gzip header from invalid(254) to UNIX(3)