KertabEncoder.write() method should take into account the size of the entries list to determine the buffer size

Currently KeytabEncoder.write() always allocate 512 bytes as the buffer size, and doesn't consider the entities list size.

ByteBuffer write( byte[] keytabVersion, List<KeytabEntry> entries )
{
  ByteBuffer buffer = ByteBuffer.allocate( 512 );
  putKeytabVersion( buffer, keytabVersion );
  putKeytabEntries( buffer, entries );
  buffer.flip();

  return buffer;
}

For each entity, KeytabEncoder.putKeytabEntry() allocates 100 buffer size.

private ByteBuffer putKeytabEntry( KeytabEntry entry )
{
  ByteBuffer buffer = ByteBuffer.allocate( 100 );
  ......
}

This mechanism fails when we create multiple principals in one keytab file.

KeytabEncoder.write() method should take into account the size of entries list to determine the buffer size. And a reasonable max size (100 currently) per entry must be determined.