handle missing "Description" in apt-cache show

handle missing "Description" in apt-cache show

do not blindly assume that all packages stanzas have a "Description:"
field in 'apt-cache show' as well as in the cache creation itself.

We instead assume now that if the stanza has a Description, it will not
be the first field as we look out for "\nDescription" to take care of
MD5sum as well as (maybe ignored) translated Descriptions embedded in
the package stanza.

Closes: #712435