Cosmetic comment fix
Problem: we need pipe creation code in a few places

The pipe in question is an inproc PAIR pipe. We use this in zactor
and also in zyre, and other places where we need a dedicated data
pipe. The code is non-trivial and easy to get wrong.

Solution: make a zsys_create_pipe () method that does this.
Problem: man pages are somewhat out of date

Solution: rebuild them (./mkmans in doc directory)
Problem: no test case for zsys_create_pipe ()

Solution: add this test case.
Merge pull request #801 from hintjens/master

Adding zsys support for pipe creation