Updated README
Problem: zlist got out of control

This was always meant to be a *simple* list container, otherwise
why would we go to all the pain of using a singly-linked list
rather than the obviously easier double-linked list... yet after
two days of refining the API while keeping the old API intact,
this thing isn't finished.

To make this worse, zlist is copying a lot from zring, except it
can't do it properly, lacking the backwards links.

Solution: simple is less, not more.

1. revert all changes to zlist and keep it as it was a week ago.

2. zring is a better place to (carefully) add new semantics.
Ziflist uses zring now
Problem: style was wrong in some places

- tabs in zsock (we really need to run a checker in Travis CI)

- zconfig_save_chunk was fugly and copied a lot of code from zconfig_save,
  so I refactored this code to make it simpler.

- also removed 1024 character limit on lines in saved config file.
Merge pull request #643 from hintjens/master

Reverting zlist changes, other cleanups