Store the actual accumulated minute length (*) in the log file just before
the cutoff value, using 'a' as separator.  This value is read back in
dcf77pi-analyze.

Add a boolean to mainloop() indicating whether the accumulated minute
length is read from the log file or calculated live.

(*) For too long minutes, the value is truncated by decode_time(). One way
to fix this probably is to store the accumulated minute length in bitinfo
instead.
Consecutive EOF markers in the log file should be skipped instead of
being considered as a valid bit. get_bit_live() never writes them.
Explain why we still need fake bitinfo values in get_bit_file().