[tls] Rename tls_session to tls_connection

In TLS terminology a session conceptually spans multiple individual
connections, and essentially represents the stored cryptographic state
(master secret and cipher suite) required to establish communication
without going through the certificate and key exchange handshakes.

Rename tls_session to tls_connection in order to make the name
tls_session available to represent the session state.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/tls.h

@@ -242,8 +242,8 @@ struct md5_sha1_digest {
 /** MD5+SHA1 digest size */
 #define MD5_SHA1_DIGEST_SIZE sizeof ( struct md5_sha1_digest )
 
-/** A TLS session */
-struct tls_session {
+/** A TLS connection */
+struct tls_connection {
 	/** Reference counter */
 	struct refcnt refcnt;