don't break STREAM socket ABI from 4.1.x to 4.2.0

Fix 1478: receive unsubscriptions in XPUB when verbose

Fixes not receiving unsubscription messages in XPUB socket with
ZMQ_XPUB_VERBOSE and using a XSUB-XPUB proxy in front.

This adds two modifications:

- It adds a new flag, ZMQ_XPUB_VERBOSE_UNSUBSCRIBE, to enable verbose
  unsubscription messages, necessary when using a XSUB/XPUB proxy.

- It adds a boolean switch to zmq::mtrie_t::rm () to control if the
  callback is invoked every time or only in the last removal. Necessary
  when a pipe is terminated and the verbose mode for unsubscriptions is
  enabled.

Fix windows build

fix #1447 Segfault or Abort after PR #1441

Fix compilation on mac.

Fix memory leak when creating a socket fails.

Previously, AddressSanitizer flagged leaks when running
tests/test_many_sockets.cpp.

Fix several missing msg_t::close calls.

I traced memory leaks found using AddressSanitizer down to these.

Fix test_proxy hanging occasionally by setting ZMQ_LINGER to 0.

Fixed #1477  corruption in "zero-copy" raw_decoder for payloads large…

Fixed wrong message::init arguments.

Fix doc formatting issues in zmq_setsockopt page

THREAD PRIORITY option documentation

Added documentation about context's options ZMQ_THREAD_SCHED_POLICY and ZMQ_THREAD_PRIORITY

Allow explicitly setting buf sizes to 0

LIBZMQ-195 allow explicitly setting sndbuf and rcvbuf to 0 (see https://support.microsoft.com/en-us/kb/201213)

on Windows, handle WSAENOBUFS like WSAEWOULDBLOCK

add message type sanity check in msg_t::rm_refs

[decoder.hpp] Fix misspelled inclusion

[decoder*] Style fixes for consistency

Addressed comments from T. Köppe

- Replaced stdlib.h with cstdlib
- Made single-argument constructors explicit

"zero-copy" raw_decoder