Warnings in Windows builds

Solution: handle types properly

Solution: Use fd_t

Solution: Use size_t instead of int

Solution: Use unsigned char instead of size_t

Solution: Use unsigned char instead of size_t

Solution: Use fd_t

Solution: Use fd_t

Solution: explicitly compare function pointer against NULL

Solution: make proper boolean expression

Solution: make proper boolean expression

Solution: move conditional include directive to precompiled.hpp

Solution: fix type specification

Solution: fix type specification

Solution: enable warnings-as-errors

Problem: zmq_poller_[add/modify] accept invalid events arguments silently

Problem: heartbeat tests are disabled

Solution: use a typedef instead

Solution: check and return an error on invalid arguments. Fixes #3088

Solution: re-enable them

Solution: extract common code into functions

Add poller docs

Solution: add symlinks to zmq_poller.txt

Solution: add initial documentation

Solution: return NULL and set errno to ENOMEM

Solution: change comment, and add test case for zmq_poller_modify with events == 0

Problem: inconsistent behaviour of zmq_poller_add and zmq_poller_add_fd in case of memory exhaustion

Solution: always return -1 with errno == ENOMEM

ZMTP 3.1 PING Context not implemented

Solution: migrate to unity assertions

Solution: if a PING message contains a context, echo it back in the
PONG message. In order to do so, create the PONG message when PING
is received and store it in the engine.
After the PING the engine goes straight to encoding and sending, so
there can always be at most one pending PING.
Add tests for various contexts.

Solution: treat the first byte of the command body as the size of the
command name, rather than as an id, to comply with ZMTP 3.1.
This was not an actual problem at runtime since both heartbeat
commands have a size of 4, which was treated like an id.
But once SUBSCRIBE/UNSUBSCRIBE get implemented it needs to be checked.

Solution: remove it, it doesn't add much value anyway

Solution: migrate to unity

Problem: building on Linux with Sun Studio still fails

Solution: initialise it

Solution: add compiler flags to support extensions like anonymous
structs in unions, fix casting of const, and check for suncc rather
than solaris

Problem: no way to specify source interface for UDP multicast sender …

Solution: if a binding interface is provided in the sender URL we pass
it to IP[V6]_MULTICAST_IF setsockopt

Problem: test_radio_dish triggers aliasing errors with some compilers

Solution: refactor the test code to work around aliasing issues