Revert "linking fd to pipe identity via socket option"

This reverts commit fe3e8c5c.

Conflicts:
	include/zmq.h
	src/pipe.hpp
	src/session_base.cpp

This is still raw and experimental.
To connect through a SOCKS proxy, set ZMQ_SOCKS_PROXY socket option on
socket before issuing a connect call, e.g.:

    zmq_setsockopt (s, ZMQ_SOCKS_PROXY,
        "127.0.0.1:22222", strlen ("127.0.0.1:22222"));
    zmq_connect (s, "tcp://127.0.0.1:5555");

Known limitations:
- only SOCKS version 5 supported
- authentication not supported
- new option is still undocumented

Metadata are built in stream_engine now.
This makes it easy to extend metadata with user-defined properties.

The get_credential () member function returns
credential for the last peer we received message for.
The idea is that this function is used to implement user-level API.

This reverts commit f27eb67e, reversing
changes made to a3ae0d4c.

https://zeromq.jira.com/browse/LIBZMQ-576

Conflicts:
	src/stream_engine.cpp

Conflicts were around additional defaults to the constructor after the
'terminating' default.  The additional defaults were left alone, and
the 'terminating' default was removed.

* Command names changed from null terminated to length-specified
* Command frames use the correct flag (bit 2)
* test_stream acts as test case for command frames
* Some code cleanups

- we need to switch to PLAIN according to options.mechanism
- we need to catch case when both peers are as-server (or neither is)
- and to use username/password from options, for client

This implements protocol handshake.
We still need to design and implement 1) API changes so a user
can set username and password, and 2) a mechanism for engine
to authenticate users.

The patch makes the code somewhat simpler and prepares it for
more complex initialization handshakes.

* Starting draft ZMTP/2.1 protocol (revision 2)
* Basis for adding security to the wire protocol
* Maintains backward compatibility

Copyrights had become ads for Sustrik's corporate sponsors, going against the original
agreement to share copyrights with the community (that agreement was: one line stating
iMatix copyright + one reference to AUTHORS file). The proliferation of corporate ads
is also unfair to the many individual authors. I've removed ALL corporate title from
the source files so the copyright statements can now be centralized in AUTHORS and
source files can be properly updated on an annual basis.

When we send a large message, the message can be splitted into two chunks.
One is in the encoder buffer and the other is the zero-copy pointer.
The session could get the term before the last chunk is sent.

Fix warnings with ICC.

This reverts commit 1a18c7b0, reversing
changes made to bef9a41b.

Significantly reworked the monitoring infrastructure with a more granular per socket API and to play well with monitoring endpoints in application threads

Since ZMQ 2.x does not support subscription forwarding, it's not
possible to use ZMQ 2.x SUB socket to receive messages from a PUB
socket.

This patch adds some compatibility layer so that ZMQ 2.x SUB socket
receives messages from PUB socket.

The new protocol adds support for protocol version and  exchanges the
socket type, so that the library can reject a connection when the
sockets do not match.

The protocol was designed so that it's possible to detect and fully
support ZTP/1.0 peers.

When a new connection is set up, peers exchange greeting messages. The
greeting message encodes both the protocol verion and the socket type.
The format of the greeting message is as follows:

    greeting    = tag1, adaptation, tag2, version, length, socket_type
    tag1        = BYTE / 0xff
    adaptation  = 8 BYTES
    tag2        = BYTE / 0x7f
    version     = BYTE / 1
    length      = BYTE / 1
    socket_type = BYTE

The protocol does not define the value of adaptation field.

When interoperability with ZTP/1.0 peers is required, the adaptaion
encodes, in network byte order, the length of identity message increased
by 1. When adaptaion consists of eight zeros, the current
implementatatio of 0MQ 2.x closes the connection.

This patch supports both ZTP/1.0 and new protocol.