mhshow: apply marker form to all part separators

mhshow:  apply marker form to all part separators

the mhshow "-markform" format is now used for all part separators, not
just those being left out.  the format now overloads the %(unseen)
function to indicate that the content of the part being marked has
been suppressed.  the built-in default and example formats both make
use of this function.