Merge pull request #488 from hintjens/master

Removed debug output in zbeacon
Merge pull request #489 from hintjens/master

Problem: need success/failure capacity in zsock signals
Merge pull request #490 from hintjens/master

Problem: zmsg_send() failed if message was null
Merge pull request #491 from hintjens/master

Problem: report when unable to get a usable interface
Removes duplicated declaration
Merge pull request #493 from marcelo-rocha/master

Removes duplicated declaration
Merge pull request #492 from hintjens/master

Added configurability for zsys global context