zbeacon: uses only interfaces that are running

Adds a check in s_get_interface to ignore the interfaces that do not
have the IFF_UP flag set.
zbeacon: Adds virtual interface "*"

If zsys_interface() is set to “*”, make the beacon broadcast to
INADDR_BROADCAST and bind to INADDR_ANY
zbeacon: Ignore interfaces that are bonding slaves

Ignore interfaces that have the IFF_SLAVE flat set, if this flag is
defined.
Merge remote-tracking branch 'upstream/master'
Merge pull request #567 from hintjens/master

Fixing CZMQ to work again on ZeroMQ 2.x
Merge pull request #568 from hintjens/master

Fixing Travis build for all versions of libzmq
Merge pull request #569 from hintjens/master

Small fixes needed for Zyre gossip work
workaround warning of non-constant static initializer
Merge pull request #570 from pijyoi/master

workaround warning of non-constant static initializer
Merge pull request #571 from hintjens/master

Small improvements
Merge pull request #573 from hintjens/master

Minor documentation changes
Merge branch 'master' of https://github.com/ahhentz/czmq
Adds monotonic clock implementation for OSX

Mac OSX does not have clock_gettime(). This adds a monotonic clock
implementation based on this stackoverflow answer

http://stackoverflow.com/questions/11680461
Merge pull request #576 from ahhentz/bugfix

Adds monotonic clock implementation for OSX
zbeacon.c: Remove unused variable in s_get_interface (mingw)

Related issues: #564
Merge pull request #578 from jtanx/win32

zbeacon.c: Remove unused variable in s_get_interface (mingw)
Define class export explicitly
Fixes to travis.yml
Fixed syntax error in .travis.yml
Problem: broke signal handling for some apps

Commit ce664b9 removed the ability for apps to override CZMQ's
handling of SIGINT/SIGTERM, which is a breaking change with no
workaround.

Solution: re-instate zsys_handler_set(NULL) to mean 'use an
external signal handler'. Current code is hopefully cleaner and
more obvious now, and this call can be used at any point to
disable CZMQ's internal signal handling.