notmuch: sync vfolder_format to folder_format

notmuch: sync vfolder_format to folder_format

The expandos used by NotMuch's vfolder_format didn't match those of
Mutt's folder_format.  This caused confusion.

vfolder_format expandos:
    %C  current file number
    %f  folder name (description)
    %m  number of messages in the mailbox *
    %n  number of unread messages in the mailbox *
    %N  N if mailbox has new mail, blank otherwise

Default setting:
    set vfolder_format = "%2C %N %?n?%4n/&     ?%4m %f"