update 'mp->lowmsg' from 0 to 1 during regular 'inc' from mail spool;

avoids failure of 'folder_realloc()' for case of no messages in
folder, and more than 100 messages to incorporate.  Initially,
'folder_read()' will initialize 'mp->lowmsg' to 0, and 'mp->lowoff'
to 1.  So the call would be folder_realloc(mp, 1, 200), and the
comparison 'if (mp->nummsg > 0 && lo > mp->lowmsg)' will fail.