Update links to https.

www.openssh.com now supports https and ftp.openbsd.org no longer
supports ftp.  Make all links to these https.
diff --git a/INSTALL b/INSTALL
index aea5790..6bc80b6 100644
--- a/INSTALL
+++ b/INSTALL
@@ -266,4 +266,4 @@
 
 If you experience problems compiling, installing or running OpenSSH.
 Please refer to the "reporting bugs" section of the webpage at
-http://www.openssh.com/
+https://www.openssh.com/