Add file descriptor support for poller

[stephanschim]

Add file descriptor support for zmq::active_poller and zmq::base_poller_t.

Note: On Windows, only socket file descriptors are supported. This is a limitation of libzmq.

[uyha]

@gummif could you have a look at this PR?

[coveralls]

Pull Request Test Coverage Report for Build 5659739096

• 39 of 43 (90.7%) changed or added relevant lines in 2 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.06%) to 86.313%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
zmq_addon.hpp	35	39	89.74%
<!--	Total:	39	43	90.7%	-->

Files with Coverage Reduction	New Missed Lines	%
zmq_addon.hpp	1	95.02%
zmq.hpp	1	82.81%
<!--	Total:	2		-->

Totals
Change from base Build 5321226093:	0.06%
Covered Lines:	845
Relevant Lines:	979

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💛 - Coveralls

[gummif]

Looks good 👍