Make hmac_ctx optional

Note from future self: actually md_init_ctx will be re-introduced with the
same signature later, and a new function with the additional argument will be
added.
diff --git a/programs/aes/aescrypt2.c b/programs/aes/aescrypt2.c
index e61ba7d..e14acae 100644
--- a/programs/aes/aescrypt2.c
+++ b/programs/aes/aescrypt2.c
@@ -101,7 +101,7 @@
     aes_init( &aes_ctx );
     md_init( &sha_ctx );
 
-    ret = md_init_ctx( &sha_ctx, md_info_from_type( POLARSSL_MD_SHA256 ) );
+    ret = md_init_ctx( &sha_ctx, md_info_from_type( POLARSSL_MD_SHA256 ), 1 );
     if( ret != 0 )
     {
         polarssl_printf( "  ! md_init_ctx() returned -0x%04x\n", -ret );
diff --git a/programs/aes/crypt_and_hash.c b/programs/aes/crypt_and_hash.c
index a1406b6..2a3f3cc 100644
--- a/programs/aes/crypt_and_hash.c
+++ b/programs/aes/crypt_and_hash.c
@@ -185,7 +185,7 @@
         polarssl_fprintf( stderr, "Message Digest '%s' not found\n", argv[5] );
         goto exit;
     }
-    md_init_ctx( &md_ctx, md_info);
+    md_init_ctx( &md_ctx, md_info, 1 );
 
     /*
      * Read the secret key and clean the command line.
diff --git a/programs/hash/generic_sum.c b/programs/hash/generic_sum.c
index f2a947e..6cb9951 100644
--- a/programs/hash/generic_sum.c
+++ b/programs/hash/generic_sum.c
@@ -204,7 +204,7 @@
         polarssl_fprintf( stderr, "Message Digest '%s' not found\n", argv[1] );
         return( 1 );
     }
-    if( md_init_ctx( &md_ctx, md_info) )
+    if( md_init_ctx( &md_ctx, md_info, 0 ) )
     {
         polarssl_fprintf( stderr, "Failed to initialize context.\n" );
         return( 1 );