Revert "minlen can never be 60 when processing DST changes, so do not check for that."

This reverts commit b65c89b367efa1f208dbbcaa98dbefaf203a937f.

Be more relaxed when processing DST changes in case of reception errors,
1/2
Also allow DST changes if announced and bit 17 is equal to bit 18.

Be more relaxed when processing DST changes in case of reception errors,
2/2
Mention the backspace key in README.md
Reset DST change announce flag at hh:00, this prevents the flag being set forever in case of a reception error.
Reset leap second announcement at 00:00 UTC, this prevents the flag being set forever in case of a reception error.