The shared reference count was not shared but copied. msg_t cannot
store the refcnt itsef but has to store a pointer to an externally
allocated (shared) refcnter. The changes to lmsg are reverted to
use content_t again. Howver, this introduces an allocation in v2_decoder
when creating the message which can be avoided. When allocating the reception
buffer, space is allocated for the maximum number of reference counts
(8192 / max_vsm_size = 8192/64 = 128 zmq:atomic_counter objects). This
increases the buffer by 128*sizeof(atomic_counter) = 128*4 = 512 bytes only.
When creating a message, the refcnt member is set to the address of one of the
pre-allocated atomic_counter_t objects. To do so, a new msg_t type zcmsg
is introduced because msg::copy must discriminate between the message types
when releasing memory.

zero-copy msg_t::init cannot be used when the message exceeds either
the buffer end or the last received byte. To detect this, the buffer
is now resized to the numnber of received bytes.

Probably fix build issue on Solaris.

See #1220 recent comments. This change is untested yet seems
safe.

Problem: 4.1 broke the ABI yet did not bump ABI number

Solution: bump to ABI version 5

Fix windows build

test zmq_msg custom free-function

new allocation caused reference counters to lose track on copy

don't check POLLOUT for single-socket proxies

Fix initialization order.

Fixes #1431.

Enable install of man pages when BUILD_DOC is not set by shifting INS…

Enable install of man pages when BUILD_DOC is not set by shifting INSTALL_MAN outside of BUILD_DOC's if/endif

Update documentation for zmq_setsockopt.

Pull request #1426 now allow for changing the watermark
after and connect() or a bind(). This patch reflect the
change in the documentation.

Also closes #1416.

Allow changing of high water marks after connection is established

Fixed executable name in the usage string

Fix xpub test to set hwm before bind.

Fail ZMQ_SNDHWM and ZMQ_RCVHWM setsockopt if already connected.

Problem: tests/test_socketopt_hwm fails and breaks CI

Solution: mark this as XFAIL

Introduce a failing test against zmq_setsockpt().

Problem: zmq_setsockpt() returns success when changing the
HWM after a bind or connect() even though the call has no effect.

Solution: Introduce a failing test a reminder we need to patch it.

Zero copy message receive in v2_decoder

Construct messages from a reference-counted buffer allocated once
per receive instead of copying the data.

With a msg_t size of 64 bytes, it becomes possible to embedd the content_t's members
struct for large messages directly in the msg_t. This saves the dynamic allocation
of content_t obejcts when using msg_t::init_data.

content_t contains a zmq::atomic_counter_t object which is not a POD in C++98
and thus it cannot be used as a member of the union u. To bypass this, C++11
is used which has relaxed rules for POD and atomic_counter is a C++11-POD. An
alternative would have been to make atomic_counter a classical POD by removing
constructors and all private member functions, i.e. have a struct and free functions
to manipulate it.

A new msg_t::init function is added which decides to either to copy the data for size<64 bytes
or use msg_t::init_data to do zero-copy initialization.

Fixes to README