Unit-test fixes and manual page updates for DoH configuration

This commit contains fixes to unit tests to make them work well on
various platforms (in particular ones shipping old versions of
OpenSSL) and for different configurations.

It also updates the generated manpage to include DoH configuration
options.

lib/isc/netmgr/http.c

@@ -790,6 +790,10 @@ http2_do_bio(isc_nm_http2_session_t *session) {
 		 * this situation.
 		 */
 		sz = nghttp2_session_mem_send(session->ngsession, &data);
+		if (sz == 0) {
+			/* No data returned */
+			return;
+		}
 		INSIST(session->r.base == NULL);
 		session->r.base = isc_mem_get(session->mctx, sz);
 		session->r.length = sz;