(get_new_msgnum): New function, loops across the list returned by

(get_new_msgnum): New function, loops across the list returned by
getfiles and returns the number for a new message to go into an mh
folder.  This is a replacement for the command 'mhpath new'.

(store_message): Address the XXX comment about using 'mhpath new' by
not using it, instead calling get_new_msgnum.  Furthermore, use an
algorithm similar to the maildir delivery algorithm to get the
filename to rename to.