mhshow-show-*: Made %e escape the default, i.e. always serialize the output.

Never invoke several content display processes in parallel, but always
wait for the previous to finish before executing the next one. The idea
behind this is generating console output only, but not spawning any
graphical windows. All output of displaying one message should go to one
single pager invocation. Any other kinds of attachments are to be stored
and afterwards displayed manually. This is the way all the other MUAs do it
and it matches the modern understanding of emails: a message and attachments.
This is contrary to the original design of MIME, true.
Also, removed the %p alias for the %l escape.