bug#44597: 26.3; bibtex should allow reverse sorting

[Francesco Potortì]

>>  Allowed non-nil values are:
>> -plain or t   All entries are sorted alphabetically.
>> +plain or t   Entries are sorted alphabetically.
>> +reverse      Entries are sorted in reverse alphabetic order.
>> +date         Entries are sorted by date.
>> +reversedate  Entries are reverse sorted by date, starting with newest.

Thinking again, maybe the reverse flag should be orthogonal to the
rest, to be applied separately?