Add isc_nm_has_encryption()

This commit adds an isc_nm_has_encryption() function intended to check
if a given handle is backed by a connection which uses encryption.

lib/isc/netmgr/netmgr-int.h

@@ -1695,6 +1695,9 @@ isc__nm_http_bad_request(isc_nmhandle_t *handle);
  * socket
  */
 
+bool
+isc__nm_http_has_encryption(const isc_nmhandle_t *handle);
+
 void
 isc__nm_http_set_maxage(isc_nmhandle_t *handle, const uint32_t ttl);