Pull values from config file before starting Zyre

Handles these values so far:

    #   Zyre cluster configuration
    zyre
        port = 5670         #   Default = 5670
        interval = 250      #   Default = 1000 msec

Also, if Zyre cannot be started, due to lack of usable network
interface, reports this and in zpipes_test_cluster, does not try
to continue tests. This tracks the latest CZMQ and Zyre masters
which provide this feedback.
Regenerated server engine (larger log prefix)

Using new zproto_server_c.gsl that increases log prefix size
Was 20 chars, is now 33 chars.

Fixes #88
Config file zyre/name specifies node name

Also: zyre/interface specifies node interface to use

Fixes #81
Fixes #89
Merge pull request #93 from hintjens/master

Pull values from config file before starting Zyre