Problem: zsock_option.c:78 fails on MSVC

Some confusion in the code, passing a polymorphic socket reference to a
method that accepts only zsock_t *.

Solution: change zsock_type_str() to accept a polymorphic reference and
resolve properly to get the socket type.
Merge pull request #651 from hintjens/master

Problem: zsock_option.c:78 fails on MSVC