add new message separator header to mhshow

add new message separator header to mhshow

new switches to mhshow (-headerform and -[no]header) control whether
and what is displayed before the display of every message's content.
the default separator looks like "[ Message folder:msg ]".  the
implementation parallels the -markform feature.

since mhshow is often started by show, show now passes the -[no]header
options through to mhshow.  (also, show no longer attempts to pass
such options through to cat (in the case of -noshowproc)).