Kernel: Add Socket::set_role() and use it everywhere

Instead of having Socket subclasses write their role into Socket::m_role
directly, add a setter to do this.

Kernel/Net/IPv4Socket.cpp

@@ -141,7 +141,7 @@ KResult IPv4Socket::listen(size_t backlog)
         return result.error_or_port.error();
 
     set_backlog(backlog);
-    m_role = Role::Listener;
+    set_role(Role::Listener);
     evaluate_block_conditions();
 
     dbgln_if(IPV4_SOCKET_DEBUG, "IPv4Socket({}) listening with backlog={}", this, backlog);