Merge pull request #809 from hurtonm/master

Simplify ypipe_t and ypipe_base_t template parameters
ZMQ_SRCFD docs and tests

Also moved the fd field from message content to message itself
typo in docs
Merge pull request #810 from sradomski/master

ZMQ_SRCFD docs and tests
Make it explicit that the probe function doesn't modify passed message