Expose zmq_setsockopt publicly

A diff for such a change might look like:
```diff
**diff --git a/src/lib.rs b/src/lib.rs
index c8c2b91..60468c8 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -31,7 +31,7 @@ macro_rules! zmq_try {
}

mod message;
-mod sockopt;
+pub mod sockopt;

use crate::message::msg_ptr;
pub use crate::message::Message;**
```
I would be happy to open a PR given feedback from maintainers about how they would like the change to be made so that it is most likely to be merged.

Start in src/lib.rs at the sockopt module declaration shown in the issue and inspect how the crate currently exposes its public API. Make the requested socket-option interface publicly accessible, then verify that the crate builds and that external callers can reach it.

Written by the indexing model from the issue text.