Implement OCSP stapling support

Currently dogfooding this patch at gemini.sgregoratto.me. To test,
run the following command and look for the "OCSP response" header:

 openssl s_client -connect "gemini.sgregoratto.me:1965" -status

1 files changed, 1 insertions, 0 deletions

diff --git a/gmid.h b/gmid.h
index ecd53a2..f7c846e 100644
--- a/gmid.h
+++ b/gmid.h
@@ -118,6 +118,7 @@ struct vhost {
 	const char	*domain;
 	const char	*cert;
 	const char	*key;
+	const char	*ocsp;
 	const char	*cgi;
 	const char	*entrypoint;