Move TLS API to tls.h, fix builtin handshake codepath

src/tls.h

@@ -31,3 +31,8 @@ void mg_tls_handshake(struct mg_connection *);
 // Private
 void mg_tls_ctx_init(struct mg_mgr *);
 void mg_tls_ctx_free(struct mg_mgr *);
+
+// Low-level IO primives used by TLS layer
+enum { MG_IO_ERR = -1, MG_IO_WAIT = -2, MG_IO_RESET = -3 };
+long mg_io_send(struct mg_connection *c, const void *buf, size_t len);
+long mg_io_recv(struct mg_connection *c, void *buf, size_t len);