(pmail): Remove a redundant message counter reset call; try to

(pmail): Remove a redundant message counter reset call; try to
minimize the attempts to show a message (ideally it should be done
once and only once).

(pmail-convert-babyl-to-mbox): Set up the global message variables
after inserting the converted mbox file into the Pmail buffer; avoid
an unecessary message display function call.

(pmail-show-message-maybe): Use the current message number when the
optional argument is not passed.