Protect callback function calls from macro substitution

Some structure fields holding callback addresses have the same name as the
underlying system function (connect, send, recv). Set parentheses around
their reference to suppress a possible macro substitution.

Use a macro for connect() on OS/400 to resolve a const/nonconst parameter
problem.

src/agent.c

@@ -688,7 +688,7 @@ libssh2_agent_connect(LIBSSH2_AGENT *agent)
     int i, rc = -1;
     for (i = 0; supported_backends[i].name; i++) {
         agent->ops = supported_backends[i].ops;
-        rc = agent->ops->connect(agent);
+        rc = (agent->ops->connect)(agent);
         if (!rc)
             return 0;
     }