net: convert standalone SetSocketNonBlocking() to Sock::SetNonBlocking()

This further encapsulates syscalls inside the `Sock` class.

Co-authored-by: practicalswift <practicalswift@users.noreply.github.com>

src/test/fuzz/util.h

@@ -80,6 +80,8 @@ public:
 
     int GetSockName(sockaddr* name, socklen_t* name_len) const override;
 
+    bool SetNonBlocking() const override;
+
     bool IsSelectable() const override;
 
     bool Wait(std::chrono::milliseconds timeout, Event requested, Event* occurred = nullptr) const override;