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Problem: man pages and selftest are slightly outdated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.