nfs4: Initial commit

Empty file system with only mounting supported.
nfs4: Add network code

Connection class supports sending and receiving RPC data using either
TCP or UDP as a transport protocol.
nfs4: Add XDR encoder and decoder

Base classes XDR::Stream and classes XDR::WriteStream and XDR::ReadStream
are added. They implement encoding and decoding XDR (RFC 4506) streams.
All data types used by RPC and NFS4 are supported.
nfs4: Add call generation code including authentication

RPC::Call generates a RPC call using provided RPC::Auth objects.
RPC::Auth supports AUTH_NONE and AUTH_SYS authentication protocols.
nfs4: Add stub RPC reply code

RPC::Reply interprets incoming RPC replies. Currently only XID is
read.
nfs4: Remove unnecessary 'Get' prefixes from accessor methods
nfs4: Add ServerAddress comparators
nfs4: Add RPC main communication code

RPC::Server represents a server client is connected to.
RPC::RequestManager stores all call that have been sent but no answer was
received so far.
RPC::ServerManager is responsible for reusing connections to the servers
and closing them when no longer needed.
nfs4: Add RPC::Server release and repair code
nfs4: Make RPC::Reply decode RPC errors
nfs4: Make mount() send null nfs4 call
nfs4: Add some NFS4 constants
nfs4: Add NFS4 request builder

Currently supports only the following operations: ACCESS, GETATTR, GETFH
LOOKUP and PUTROOTFH.
nfs4: Add NFS4 reply interpreter

Currently supported operations are: ACCESS, GETATTR (partially), GETFH,
LOOKUP and PUTROOTFH.
nfs4: No need to cast from enum to int
nfs4: Fix rpc listener thread cancelation

recv on UDP sockets is not interrupted when the socket is closed.
Additionaly, signals do not interrupt kernel threads, that is why
new function interrupt_thread was implemented in order to cancel
listener thread.
nfs4: Add stub Filesystem class and Mount method
nfs4: Add stub Inode class, fix and extend Filesystem class

Additionally PUTFH support is added to both RequestBuilder and
ReplyInterpreter.
nfs4: Implement nfs4_mount() procedure
nfs4: Add basic nfs4_read_stat() procedure

Not all data are retrieved and there is a room for improvement in handling
situations when server does not provide necessary information.