Fix compilation warnings on unsigned comparisons.

Fix two unsigned comparisons to zero or more being always true. Clang
won't compile this with -Werror.
Merge pull request #607 from cjuniet/master

[LIBZMQ-544] Fix compilation errors with Clang
Merge pull request #606 from ckamm/master

Add tests for Request-Reply and Pipeline pattern sockets.
REQ sockets drop replies from unasked peers.

* Add lb_t::sendpipe() that returns the pipe that was used for sending,
  similar to fq_t::recvpipe().
* Add forwarder functions to dealer_t to access these two.
* Add logic to req_t to ignore replies on pipes that are not the one
  where the request was sent.