Added support for referencing a message on the command-line with the
'l' (list messages), 'x' (dump index), 'r' (read msgs), and 'v' (view msgs) commands by "days old", by specifying a negative number (in days). (e.g. "smbutil r-30" would read messages posted within the past 30 days). This was added to test the smb_getmsgidx_by_time() function, but has some value otherwise I suppose.