Merge pull request #780 from hashstat/master

Shortened ZMQ_IPC_ACCEPT_FILTER_[UGP]ID to ZMQ_IPC_FILTER_[UGP]ID.
Enable building IPC filtering with cmake.
Move {SO,LOCAL}_PEERCRED checks to C++ context.
Merge branch 'master' of https://github.com/zeromq/libzmq