Require manual override for user chain certificates.

An override for the wallet to daemon connection is provided, but not for
other SSL contexts. The intent is to prevent users from supplying a
system CA as the "user" whitelisted certificate, which is less secure
since the key is controlled by a third party.

contrib/epee/include/net/net_ssl.h

@@ -51,7 +51,8 @@ namespace net_utils
   {
     none = 0,         //!< Do not verify peer.
     system_ca,        //!< Verify peer via system ca only (do not inspect user certificates)
-    user_certificates //!< Verify peer via user certificate(s) only.
+    user_certificates,//!< Verify peer via specific (non-chain) certificate(s) only.
+    user_ca           //!< Verify peer via specific (possibly chain) certificate(s) only.
   };
 
   struct ssl_authentication_t