handle clean SSL connection shutdowns

some servers actually bother to close down the SSL connection before
closing the socket.

this fixes the spurious "unhandled SSL error 6" messages.

REFMAIL: 20150120114805.GA17586@leeloo.kyriasis.com

src/socket.c

@@ -76,6 +76,7 @@ ssl_return( const char *func, conn_t *conn, int ret )
 	case SSL_ERROR_SSL:
 		if (!(err = ERR_get_error())) {
 			if (ret == 0) {
+	case SSL_ERROR_ZERO_RETURN:
 				/* Callers take the short path out, so signal higher layers from here. */
 				conn->state = SCK_EOF;
 				conn->read_callback( conn->callback_aux );