Redesign the calculations for acc_minlen.

The old calucations were imprecise and sometimes off. For the new
calculations:
- minute lengths are only reset at an EOM marker
- in decode_time():
  - use a working copy of acc_minlen
  - partial minutes are summed and when at least 60,000 , added to the
    working copy of acc_minlen
  - if the previous minute was too long, increase with
    acc_minlen - old_acc_minlen ms instead of acc_minlen ms
  - for overflowing minutes, acc_minlen is first estimated by get_bit_file().
    This can result in too high estimations, but this is fixed by
    the above substraction.
  - the sum of the partial acc_minlen never reset to 0, but reset to its
    value % 60000 to better keep track of time.
  - get rid of variable toolong.

Tested with old log files, which now behave the same as live input except for
the above estimation.
Use 0xff instead of 61 as a signaling value for GETBIT_TOOLONG.
Sort variables in decode_time on size.
Remove obsolete add_acc_minlen().
Fix overflows in the calculations here too.
Replace 6"PRIi64" with regular 6u since the values are now 32 bits.
Fix the 'Q' button in dcf77pi. No idea why this is needed...