Solution: rename socket option (and variables and files) from usefd
to use_fd.

Solution: rename socket option (and variables and files) from
pre_allocated_fd to usefd.

Solution: parse the value set by the ZMQ_PRE_ALLOCATED_FD sockopt
when creating a new TCP socket and use it if valid.
Add new tests/test_pre_allocated_fd_tcp.cpp unit test.

Solution: parse the value set by the ZMQ_PRE_ALLOCATED_FD sockopt
when creating a new IPC socket and use it if valid.
Add new tests/test_pre_allocated_fd_ipc.cpp unit test.

This commit fixes the author warnings reported below. These tests
have originally been introduced in zeromq/libzmq@5ebfd17 and
zeromq/libzmq@5fe75f0.

8<---8<---8<---8<---8<---8<---8<---8<---8<---8<---8<---
CMake Warning (dev) at tests/CMakeLists.txt:133 (message):
  Test 'test_udp' is not known to CTest.
This warning is for project developers.  Use -Wno-dev to suppress it.

CMake Warning (dev) at tests/CMakeLists.txt:133 (message):
  Test 'test_large_msg' is not known to CTest.
This warning is for project developers.  Use -Wno-dev to suppress it.
8<---8<---8<---8<---8<---8<---8<---8<---8<---8<---8<---

found.
Solution: Include the source directory

Problem: _sleep is a library function of Windows.
Solution: use sleep_

VMCI transport allows fast communication between the Host
and a virtual machine, between virtual machines on the same host,
and within a virtual machine (like IPC).

It requires VMware to be installed on the host and Guest Additions
to be installed on a guest.

Solution: add test case

This commit adds ZMTP connection heartbeats described in
http://rfc.zeromq.org/spec:37/ZMTP.

new allocation caused reference counters to lose track on copy

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.

added version to builds
other cleanup

ZMQ_INVERT_MATCHING reverses the PUB/SUB prefix matching. The subscription
list becomes a rejection list. The PUB socket sends messages to all
connected (X)SUB sockets that do not have any matching subscription.

Whenever the option is used on a PUB/XPUB socket, any connecting SUB
sockets must also set it or they will reject everything the publisher
sends them. XSUB sockets are unaffected because they do not filter out
incoming messages.