Problem: selftest leaked memory when networking was off

The selftest exits when it can't create a beacon; it didn't destroy
the context and so leaked memory. Fixed.
Problem: unnecessary delays in zauth selftest

Solution: reduced failure delay from 1,000 msec to 200 msec.
Reordered methods for better clarity
Problem: zhash_foreach() is clumsy and horrible

Callbacks are nasty, and I much prefer using the zlist first/next
style iterators, so I've added this to zhash and it works nicely.

Solution: use simple iterator API as learned from zlist, so caller
can get items from hash table in some arbitrary order.

Difficulty: we need to provide both key and value, so the methods
return the value, and a helper method (zhash_cursor) to return
the key.
Removed dangling debug output
Problem: 'service' is overloaded in zgossip

Solution: abstract the gossip model away from service discovery
(implemented as services), to a more general tuple gossip model.
In this, services exchange name-value tuples, that can be used
for discovery, configuration data, etc. This makes it much easier
to understand. The gossip protocol operates independently of the
data it is carrying.

Also, now sends tuples to new remote server upon connection.
Merge pull request #513 from hintjens/master

More changes for zgossip and other improvements
Feature: Added more syslog functionality to zlog
Merge pull request #515 from mvala/master

Feature: Added more syslog functionality to zlog
Switched to using zhash_first/next iterators for tuples

Much nicer...! I really like using this iterator style, keeping the
cursor context in the collection.
Adding service name for tracebilty
Revert "Feature: Added more syslog functionality to zlog"

This reverts commit b289d0622e7ce10cbcb5d6e9e13eb0bb6dc60119.

Mostly because complex constructors are an anti-pattern; it is
nicer to configure an object by setting properties on it. Also
because zlog was abstracted over syslog, and this change breaks
the abstraction and makes it specific to syslog.
Merge pull request #516 from hintjens/master

Minor improvements
Removed debug output
Merge pull request #517 from hintjens/master

Removed debug output
Updated man page for zgossip
Problem: zconfig crashed when hitting invalid config file

After loading chunk, did not check properly for nullified
self, signalling invalid file.

Solution: check properly for this. Also, fixed up error
messages to use zclock_log instead of fprintf.
Regenerated man pages
Added temporary tracing for zsys_set_interface
Problem: logging is not consistent

Solution:

* Remove (kill) zlog class, which turns out to be too hard to use as
  it requires passing class instance all around application.

* Deprecated zclock_log method, which is not an obvious place for a
  default logging method.

* Added zsys_error, zsys_warning, zsys_notice, zsys_info, zsys_debug
  to send log messages to system facility.

* Added zsys_set_logident to define prefix or sender name for searching.

* Added zsys_set_logstream to allow logs to be sent to stdout/stderr.

* Added support for ZSYS_LOGIDENT and ZSYS_LOGSTREAM=stdout/stderr in
  the environment.