fix various issues

- rename function and variable
- change signature of `ssl_tls13_has_configured_psk`
- remove unnecessary statements
- remove unnecessary local variables
- wrong variable initial value
- improve output message

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/ssl_tls13_server.c b/library/ssl_tls13_server.c
index 6e754a3..08f48b8 100644
--- a/library/ssl_tls13_server.c
+++ b/library/ssl_tls13_server.c
@@ -186,10 +186,10 @@
     if( now < session->start )
     {
         MBEDTLS_SSL_DEBUG_MSG(
-            3, ( "Ticket expired: start is in future "
-                    "( now=%" MBEDTLS_PRINTF_LONGLONG
-                    ", start=%" MBEDTLS_PRINTF_LONGLONG " )",
-                    (long long)now, (long long)session->start ) );
+            3, ( "Ticket expired: Invalid ticket start time "
+                     "( now=%" MBEDTLS_PRINTF_LONGLONG
+                     ", start=%" MBEDTLS_PRINTF_LONGLONG " )",
+                 (long long)now, (long long)session->start ) );
         goto exit;
     }