Merge pull request #690 from mattconnolly/master

config for libsodium in alternate path, using CPP flags for both C and C...
add zmq_curve_keypair to zmq_utils
use zmq_curve_keypair in curve_keygen
return NULL and set EINVAL on bad z85 input

asserts aren't appropriate for checking user input.
Merge pull request #691 from minrk/curve_keygen

Expose zmq_curve_keypair via zmq_utils
Packaging for zmq_curve_keypair function

* Added new man page for this
* Added test case, in tests/test_security_curve.cpp
* Noted in zmq_utils.h that these methods are documented
Merge pull request #692 from hintjens/master

Packaging for zmq_curve_keypair function
fix minor typo in zmq_ctx_get doc example
Merge pull request #693 from calid/doc-fix

fix minor typo in zmq_ctx_get doc example
Clarified that zmq.h constants are part of the public contract
Disabled randomly failing part of test
Merge pull request #694 from hintjens/master

Clarified that zmq.h constants are part of the public contract
Stop curve handshake when cookie box verification fails
replace macro constants with enum types for user facing constants
Merge pull request #695 from hurtonm/master

Stop curve handshake when cookie box verification fails
distinguish between options and defaults
Merge pull request #696 from calid/enum-constants

replace macro constants with enum types for user facing constants
test for _MSC_VER before using MSVC specific code
WinSock2.h filename should be all lowercase
Merge pull request #697 from pijyoi/master

bugfixes for mingw compilation