No need to change 'utchour' at this point, it won't be read again after here.
Doxygen fix for add_minute().
Add a flag to add_minute() to indicate whether ANN_CHDST should be checked.

Not setting this flag might be useful in simulations.
Add a subtract_minute() function, analogous to add_minute().
Fix an overflow resulting in truncated acc_minlen values.
Minor cleanup.
Do not immediately return from get_bit_file() upon EOF but only set the
EOD flag. Together with testing for EOD at the end of mainloop() the
situation where acc_minlen is 1000 short at the end of the file (with
trailing newline) is now fixed.
Fix commit 4b12212bcc2546edaf5d023033742fe1510fcd49
Also calculate newtime when the time increase is negative.