Introduce custom TLS init function

src/tls.h

@@ -11,6 +11,7 @@ struct mg_tls_opts {
   const char *certkey;    // Certificate key
   const char *ciphers;    // Cipher list
   struct mg_str srvname;  // If not empty, enables server name verification
+  int (*ifn)(struct mg_tls_opts *);  // Custom init func. Return 0 on success
 };
 
 void mg_tls_init(struct mg_connection *, struct mg_tls_opts *);