prevent duplicate connections from PUB sockets also

[WallStProg]

(see #788)

The original fix for the duplicate connect issue only covered SUB connecting to PUB, but the same problem exists in the other direction. (Irrespective of any other issues related to PUB connecting to SUB).

Note to Luca -- let me know if you need/want a test pgm for this, or if this behavior should be documented somewhere (e.g.in zmq_connect.txt?)

[bluca]

Thanks! A PGM test sounds good - will be useful in any case. Adding more details to the docs sounds good as well.

[WallStProg]

I meant test program, not PGM test ;-) I'll look at whipping something together -- assume that should go in tests directory. FWIW, I don't see anything there now that tests this feature.

As for docs, I'll put something in zmq_connect.txt, then if you want to move it elsewhere you can.

[bluca]

Yes the test directory is the right place. Can it be self-contained (provided libpgm is installed) or does it need some special setup in the system?

[WallStProg]

Sorry it's taken so long ... finally had time to put together doc changes and a small test program -- please see #3117