when updating MCB to add UMB chain, redetermine top of memory again in case changed (e.g. device driver relocated EBDA) to avoid corrupt MCB chain.  Revert checking for switches only on pass==0, causes error due to portion of command line not skipped when reevaluated on next pass.  Allow undocumented -1 for /E: switches option for use calculated EBDA size when moving.