Problem: zsock_fd and zsocket_fd don't work on Win64

Solution: use/return SOCKET type instead of int.

Fixes #825
Regenerated documentation
Merge pull request #844 from hintjens/master

Fixed zsock_fd and zsocket_fd return type
PROBLEM: zring was removed, but can't be implemented using zlistx and zhashx

zring has been removed recently because it's a weird data structure.
Yet, I have found it really useful, so tried to implement it on top
of zlistx and and zhashx, by storing list handles in the hash.
However, when deleting an item from the hash, one needs to convert a
list handle to the item stored in it, which is currently not possible,
because the pointer is opaque.

SOLUTION: add a function to convert a handle into the associated item.
Merge pull request #845 from skaes/convert-list-item-handle-to-list-item

PROBLEM: zring was removed, but can't be implemented using zlistx and zhashx
Problem: can't delete items while iterating

Solution: when deleting item under cursor, move cursor to previous
item so that _next works as expected.

Also, add test case for this.
Problem: zlistx_handle () is not an obvious method name

Solution: since 'cursor' is already in the API vocabulary, and this
method returns the cursor, a better name is zlistx_cursor ().
Problem: man pages and selftest are slightly outdated

Solution: regenerate/fix these.
Merge pull request #846 from hintjens/master

Small improvements to zlistx