Merge pull request #516 from hintjens/master

Minor improvements
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.
Removed temporary debug script