mingw fix: DLL_EXPORT -> LIBCZMQ_EXPORTS

since commit 5dc41fe9a16d722fb675bbd3abe4c9f59c74d0e6,
DLL_EXPORT was renamed to LIBCZMQ_EXPORTS.
fix issue 572: match socket xor fd

previously a user could populate into zmq_pollitem_t
- a non-NULL libzmq socket
- a non-zero fd
and the code would search for both types.
this logic also precluded using fd==0

the new semantics only match for fd field if socket field is invalid.
i.e. socket==NULL
Merge pull request #590 from pijyoi/master

mingw fix: DLL_EXPORT -> LIBCZMQ_EXPORTS
Merge pull request #591 from pijyoi/issue572

fix issue 572: match socket xor fd
Fix code style in zinterface
Fix memory leak in zinterface_list
Use prime numbers for hashtable sizes.
Split out hash table resizing into static function.
Add limit on the chain length in hash tables.
Document table design.
Merge pull request #579 from hintjens/master

Problem: zauth and zbeacon don't work with new zsock API
Merge pull request #593 from mrvn/pull-prime_table_size

Pull prime table size
Merge pull request #592 from ahhentz/zinterface

Fix style + bugfix in zinterface
Code cleanups

- please do not use Tabs for indentation (read CLASS style guide)
- moved long list of prime numbers to separate header file for ease
  of reading zhash.c.
Fixed include path for czmq.h
Rewrote zmonitor to use zactor

- renamed existing zmonitor -> zmonitor_v2
- renamed zsock_monitor -> zmonitor
- related cleanups in zauth