avoid extra copy of data buffer in nfs3.read

for the care where # of bytes read == count the copy is definitely not required.
i dont know if nfs allows returning a buffer larger than the amount of actual data in it (on the one hand there is a count field in the response, on the other the rfc says nothing) - so i left the copy in place for those cases. however, i think that bytesRead==count should be fairly common for files larger then the max buffer size (32k by default).

i've also removed an unused UnixUser.