Make md_info_t an opaque structure

- more freedom for us to change it in the future
- enforces hygiene
- performance impact of making accessors no longer inline should really be
  negligible
diff --git a/library/x509.c b/library/x509.c
index 174e32d..67359a5 100644
--- a/library/x509.c
+++ b/library/x509.c
@@ -845,8 +845,8 @@
         mgf_md_info = md_info_from_type( pss_opts->mgf1_hash_id );
 
         ret = polarssl_snprintf( p, n, " (%s, MGF1-%s, 0x%02X)",
-                              md_info ? md_info->name : "???",
-                              mgf_md_info ? mgf_md_info->name : "???",
+                              md_info ? md_get_name( md_info ) : "???",
+                              mgf_md_info ? md_get_name( mgf_md_info ) : "???",
                               pss_opts->expected_salt_len );
         SAFE_SNPRINTF();
     }