Removed PRINTF validation from man pages

- was mucking up man page contents
- moved to end of header file, outside API section
- simplified macros in czmq_prelude.h
Added sys_daemonize() method
Added zchunk_consume / exhausted

This allows an application to merge chunks together in an efficient
manner, without copying.
Added selftest for zchunk_exhausted
Merge pull request #418 from hintjens/master

Added zsys_daemonize method