   Convert external TAI64N timestamps to fractional seconds since epoch.

   Written by Russ Allbery <rra@stanford.edu>
   This work is in the public domain.

   Usage:

        tai64nfrac < input > output

   Expects the input stream to be a sequence of lines beginning with @, a
   timestamp in external TAI64N format, and a space.  Replaces the @ and the
   timestamp with fractional seconds since epoch (1970-01-01 00:00:00 UTC).
   The input time format is the format written by tai64n and multilog.  The
   output time format is expected by qmailanalog. 
