Problem SIGBUS under 64-bit SunOS Sparc

Solution: force the compiler to make the atomic_counter_t alignment
friendly.
This will ensure that the pointers inside the buffers allocated by
shared_message_memory are aligned, at the cost of growing the memory
size of atomic_counter_t from 4 to 8 bytes on 64 bit (when not using
mutexes).
Note that although content_t contains an atomic_counter_t, the
compiler already padded the struct so there is no change in the
buffer sizes used by the engines, save for the extra 4 bytes for the
buffer's own single atomic counter.
Fixes #2588

Solution: remove requirement to manually define macro and just check
for the C++ supported version.
Note that compiler intrinsics still have priority if available, to
avoid changes unless necessary.

CURVE throughput performance testing

Solution: Use a double for messages per second and calculate
Megabits per second using it.  Truncate messages per second
only in the reported output.

Solution: Add extra argument to enable CURVE, use
fixed keys to make local_thr and remote_thr compatible
with CURVE enabled.

Problem: CMake build does not allow static linking libsodium

Solution: Pass along SODIUM_STATIC define if set in CMake config

Problem: log for bad ZAP status code is confusing

Solution: log that handler sent bad status code to clarify ZAP debugging

Solution: implement get_endpoint in the NORM and PGM engines too

* Problem: missing test for status code 300, inadequate assertion for status code 500

Solution: add test, change assertion (currently test fails)

* Problem: gcc compiler error deprecated conversion from string constant

Solution: declare variable as const

* Problem: in case of ZAP handler returning a status code other than 200, no appropriate event is emitted

Solution: immediately emit event after receiving reply from ZAP handler

* Problem: endpoint address is not included in zap-reply monitor event

Solution: added functions to retrieve endpoint address in zmq::i_engine and zmq::session_base_t
removed unused code block in zmq::stream_engine_t::next_handshake_command

* Problem: wrong formatting

Solution: fix formatting

* Problem: test fails because of EPIPE

Solution: add EPIPE/ECONNRESET/ECONNAGAIN handling for more test cases

* Problem: CI failure on Appveyor with err==ECONNRESET

Solution: ignore ECONNRESET analogous to EPIPE

* Problem: blocking test execution

Solution: added more debug output, do not wait for monitor infinitely without output

Problems: test_security_curve occasionally hangs or fails due to ECONNABORTED

Problem: failing assertion on ZAP protocol error

Problem: occasional test failures due to messages not (yet) received in test assertion

Solution: do not assert on ZAP protocol error when connection was closed by client

Solution: add sleep

Solution: expect that error, which can happen on very slow machines
due to a client timimng out or RST before the connection is accepted
by the server

Solution: refactor the ZAP thread to avoid creating the socket in the
main thread and using it in the ZAP thread, and also to eliminate
races at shutdown use another inproc control socket to signal
termination to the thread

Problem: getrandom usage breaks build

Solution: add missing flags parameter

Problems: use-before-initialise error in print_backtrace, test_security_curve sometimes fails due to unexpected ECONNRESET

Solution: ignore ECONNRESET as with EPIPE - it can happen on very
slow machines when the engine sends data to the peer and then tries
to read from the TCP socket before the peer has read

Solution: fix them

Problems: timeout in curve test, redundant windows and android CI jobs

Problem: zmq::curve_server_t::produce_error sends sizeof std::string instead of status code length

Solution: remove it, and test it only on Linux, as the toolchain and
build process is the same.

Solution: bump timeout from 20 to 60 seconds

Solution: remove it. No need to do an additional sodium on - curve
off test, as sodium will be disabled anyway

Problem: open TODOs in test code, CI failures

Solution: send status code length (always 3) instead

Solution: add workaround at another place, unify two code fragments to remove duplication

Solution: removed code duplication
improved global variable naming
added assertions on number of ZAP requests handled
added assertion on monitor event to test_curve_security_with_plain_client_credentials

Problems: too many OSX travis builds, curve test uses hard-coded TCP port

Problem: new ZMQ_MSG_PROPERTY_* defines were not mentioned in the documentation

Problem: sporadic failure in test_curve_security_with_null_client_credentials

Solution: add documentation

Problem: Message property names are duplicated