Split the maximum fragment length into two - an input and output MFL

Since the server might want to have a different maximum fragment length
for the outgoing messages than the negotiated one - introduce a new way of
computing it. This commit also adds additional ssl-opt.sh tests ensuring
that the maximum fragment lengths are set as expected. 
mbedtls_ssl_get_max_frag_len() is now a deprecated function,
being an alias to mbedtls_ssl_get_output_max_frag_len(). The behaviour
of this function is the same as before.
Signed-off-by: Andrzej Kurek <andrzej.kurek@arm.com>
diff --git a/library/ssl_msg.c b/library/ssl_msg.c
index 18fa555..b4902be 100644
--- a/library/ssl_msg.c
+++ b/library/ssl_msg.c
@@ -214,7 +214,7 @@
     size_t max_len = MBEDTLS_SSL_OUT_CONTENT_LEN;
 
 #if defined(MBEDTLS_SSL_MAX_FRAGMENT_LENGTH)
-    const size_t mfl = mbedtls_ssl_get_max_frag_len( ssl );
+    const size_t mfl = mbedtls_ssl_get_output_max_frag_len( ssl );
 
     if( max_len > mfl )
         max_len = mfl;